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ABSTRACT Studies on preventing antimicrobial resistance in aquaculture emphasise the need to responsibly and prudently use antimicrobials, selecting those most effective in controlling and/or reducing mortalities caused by vibriosis. In this study, the distribution of the antimicrobial susceptibility of 28 Vibrio anguillarum isolates from Turkish fish farms was determined using the broth microdilution minimum inhibitory concentration (MIC) test according to the Clinical and Laboratory Standards Institute (CLSI) guideline. The epidemiological cut‐off (COWT) values of the V. anguillarum isolates of florfenicol (FLO), tetracycline (TET), doxycycline (DOX), oxolinic acid (OXO), enrofloxacin (ENR) and trimethoprim/sulfamethoxazole (SXT) were calculated using the normalised resistance interpretation (NRI) and ECOFFinder methods. Isolates were categorised as belonging to the fully susceptible wild‐type (WT) or non‐wild‐type (NWT) populations. Calculated COWT values (in μg mL−1) were ≤ 1.0 for FLO and DOX, ≤ 0.5 for TET, ≤ 0.016 for ENR, ≤ 0.032 for OXO, and ≤ 4.0 for SXT using the NRI analysis. Percentages of V. anguillarum isolates categorised as belonging to the NWT population were small for the antimicrobials FLO (10.7%) and SXT (0%), whereas they were higher for the antimicrobials OXO (39.3%) and ENR (39.3%). This is the first study to estimate the local COWT values for antibiotics used in the control of V. anguillarum isolates recovered from farmed fish in Turkey. [ABSTRACT FROM AUTHOR]

Opportunistic fish pathogen Vibrio scophthalmi frequently infects olive flounder and turbot, which are primary marine species cultured for seafood production in Far East Asia. These infections cause substantial yield reductions and significant economic losses. Although quorum sensing (QS) genes were previously reported in V. scophthalmi, the impacts of QS on genome-wide gene expression and consequent behaviors and physiological traits have remained largely unexplored. In this study, we conducted genomic and transcriptomic analyses to uncover the global regulatory network governed by LuxRVs, a QS master regulator in V. scophthalmi. By comparing the wild-type strain and a luxRVs deletion mutant strain, we found that LuxRVs positively regulates biosynthetic genes for poly-hydroxyalkanoate (PHA) while negatively controlling genes for biofilm formation. Quantification of intracellular PHAs and biofilm biomass on borosilicate tubes confirmed these results. Gene set enrichment analyses further demonstrated that LuxRVs also governs genes related to osmoprotection and defense against reactive oxygen species. Overall, these findings indicate that LuxRVs acts as a global transcriptional regulator, controlling a wide range of physiological processes in V. scophthalmi. Targeting LuxRVs could therefore be a promising strategy for improving seafood production by disrupting diverse physiological and pathogenic traits in this fish pathogen. [ABSTRACT FROM AUTHOR]

Type III secretion system (T3SS) is an important virulence system in Gram‐negative bacteria. In this investigation, different environmental conditions that regulate the expression of T3SS genes in Yersinia ruckeri were investigated aimed at obtaining a better understanding about its modulation after various environmental challenges. Four isolates of Y. ruckeri CSF007‐82, ATCC29473, A7959‐11 and YRNC10 were cultivated under the diverse in vitro challenges iron depletion, high salt, low pH and in the presence of fish serum or in the fish cell culture (Chinook Salmon Embryo – CHSE). The transcriptional modulation of the chromosomal genes ysaV, ysaC, ysaJ and prgH of ysa were investigated using quantitative real‐time PCR. The expression of prgH, ysaV, ysaC and ysaJ was differentially expressed in all four strains under evaluation. The highest gene expression levels were observed for Y. ruckeri YRNC10 AN after addition of 0.3 M NaCl in Luria Bertani broth. The results obtained from this study provide initial insights into T3SS responses in Y. ruckeri, which pave the way for further studies aimed at expanding our knowledge on the functional roles of the T3SS genes in Y. ruckeri. [ABSTRACT FROM AUTHOR]

The indiscriminate use of antibiotics in aquaculture has led to the emergence of resistance; hence, eco-friendly, host-specific alternatives to mitigate bacterial infections have become imminent. In this study, bacteria that could possibly serve as probiotics were isolated and evaluated for their efficacy with in vitro experiments and in vivo zebrafish gut model. One isolate from each of the 23 rohu fish (Labeo rohita) was shortlisted after preliminary screening of several isolates and tested for their ability to inhibit two important warm water bacterial fish pathogens, Aeromonas hydrophila, and Edwardsiella tarda. An isolate (RODK28110C3) that showed broad-spectrum inhibitory activity against a battery of different isolates of the two fish pathogens included in this study and maintained in our repository was selected for further characterization. The culture was identified phenotypically as Bacillus subtilis and confirmed by 16S rDNA sequencing. The isolate was able to hydrolyze fish feed constituents that include starch, protein, and cellulose. Further in vitro tests ensured that the potential isolate with probiotic attributes could tolerate different gut conditions, which included a range of pH, salinity, and varying concentrations of bile salt. Exposure of 4 days post fertilization zebrafish embryos to the RFP-tagged isolate confirmed the colonization of B. subtilis in the gut of the zebrafish embryo, which is an important attribute of a probiotic. The isolate was able to inhibit both A. hydrophila and E. tarda in gnotobiotic zebrafish embryo in triplicate. The study demonstrates the probiotic characteristics of the B. subtilis isolated from L. rohita and its ability to inhibit A. hydrophila and E. tarda using in vitro conditions and in the zebrafish gut and could serve as an effective alternative to antibiotics in aquaculture. [ABSTRACT FROM AUTHOR]

Abstract Opportunistic fish pathogen Vibrio scophthalmi frequently infects olive flounder and turbot, which are primary marine species cultured for seafood production in Far East Asia. These infections cause substantial yield reductions and significant economic losses. Although quorum sensing (QS) genes were previously reported in V. scophthalmi, the impacts of QS on genome-wide gene expression and consequent behaviors and physiological traits have remained largely unexplored. In this study, we conducted genomic and transcriptomic analyses to uncover the global regulatory network governed by LuxR Vs , a QS master regulator in V. scophthalmi. By comparing the wild-type strain and a luxR Vs deletion mutant strain, we found that LuxR Vs positively regulates biosynthetic genes for poly-hydroxyalkanoate (PHA) while negatively controlling genes for biofilm formation. Quantification of intracellular PHAs and biofilm biomass on borosilicate tubes confirmed these results. Gene set enrichment analyses further demonstrated that LuxR Vs also governs genes related to osmoprotection and defense against reactive oxygen species. Overall, these findings indicate that LuxR Vs acts as a global transcriptional regulator, controlling a wide range of physiological processes in V. scophthalmi. Targeting LuxR Vs could therefore be a promising strategy for improving seafood production by disrupting diverse physiological and pathogenic traits in this fish pathogen.

Abstract Background Streptococcus suis (S. suis) is an important swine and human pathogen. A recent study reported the first isolate of S. suis capable of infecting fish, designated as S. suis strain 3112. The bacterium was isolated from snakeskin gourami (Trichopodus pectoralis), an economically important fish species native to Southeast Asia, and it was previously shown that it can infect and cause lethal streptococcosis in the fish. Results In this study, we present the complete genome of S. suis 3112. Molecular sequence analysis revealed that it belongs to serotype 6, sequence type 2340. Phylogenetic analysis showed that the bacterium clustered with healthy-pig S. suis isolates, suggestive of an ultimate swine (as opposed to human) origin of the bacterium. Two fluoroquinolone resistance genes are present in the bacterial genome, namely patA and patB. Our results showed that both genes are expressed in our bacterium, and the bacterium is resistant to norfloxacin, but is still sensitive to other fluoroquinolones, including ciprofloxacin, enrofloxacin, and sparfloxacin. Additionally, the bacterium is sensitive to β-lactams, tetracyclines, sulphonamides, and an aminoglycoside. Conclusions This study reports and describes the complete genome of S. suis 3112, the first isolate of S. suis known to infect fish, and provides further insights into the bacterial isolate, particularly regarding its drug resistance profile. These results will facilitate further investigations of the comparative genomics and pathogenic characteristics of S. suis, as well as the development of control strategies against this newly-identified fish pathogen.

A strategy for vaccine design involves identifying proteins that could be involved in pathogen–host interactions. The aim of this proteomic study was to determine how iron limitation affects the protein expression of Tenacibaculum dicentrarchi, with a primary focus on virulence factors and proteins associated with iron uptake. The proteomic analysis was carried out using two strains of T. dicentrarchi grown under normal (control) and iron‐limited conditions, mimicking the host environment. Our findings revealed differences in the proteins expressed by the type strain CECT 7612T and the Chilean strain TdCh05 of T. dicentrarchi. Nonetheless, both share a common response to iron deprivation, with an increased expression of proteins associated with iron oxidation and reduction metabolism (e.g., SufA, YpmQ, SufD), siderophore transport (e.g., ExbD, TonB‐dependent receptor, HbpA), heme compound biosynthesis, and iron transporters under iron limitation. Proteins involved in gliding motility, such as GldL and SprE, were also upregulated in both strains. A negative differential regulation of metabolic proteins, particularly those associated with amino acid biosynthesis, was observed under iron limitation, reflecting the impact of iron availability on bacterial metabolism. Additionally, the TdCh05 strain exhibited unique proteins associated with gliding motility machinery and phage infection control compared to the type strain. These groups of proteins have been identified as virulence factors within the Flavobacteriaceae family, including the genus Tenacibaculum. These results build upon our previous report on iron acquisition mechanisms and could lay the groundwork for future studies aimed at elucidating the role of some of the described proteins in the infectious process of tenacibaculosis, as well as in the development of potential vaccines. [ABSTRACT FROM AUTHOR]

Peracetic acid (PAA) has a long history as an efficacious and eco‐friendly disinfectant. It was first synthesised in 1902, and since then a wide range of applications has been developed in various industries. Aquaculture is a more recent industry wherein the potential of PAA is significant. As the global demand for sustainable development increases, there has likewise been growing interest in using PAA in aquaculture as an alternative to less environmentally friendly practices. PAA has no carcinogenic risk to humans (unlike formalin), has negligible harmful by‐products (unlike chlorine‐based disinfectants) and with appropriate precautions, the risks of causing severe human health damage is easier to control than ozone. Fish show strong physiological recovery and adaptation to PAA, whereas susceptible life stages of pathogens are highly vulnerable, enabling a safe and efficacious disinfection of the entire culture water and not the flow‐restricted disinfection by such processes as ultraviolet radiation or ozone. The effective concentration of PAA against many fish pathogens is usually below 2 mg L−1, which is tolerable for most fish, and it has very low environmental risk due to rapid degradation. However, such degradation and the hydrodynamics in production‐scale aquaculture systems complicate the practical use of PAA. In this review, we summarise key results of safe concentrations of PAA and its effectiveness specifically for fish farmers. We also outline major difficulties and possible solutions for practical uses of PAA. We intend to bring global attention to this compound and inspire future possibilities for its sustainable use as a water disinfectant in aquaculture. [ABSTRACT FROM AUTHOR]

Background: Streptococcus suis (S. suis) is an important swine and human pathogen. A recent study reported the first isolate of S. suis capable of infecting fish, designated as S. suis strain 3112. The bacterium was isolated from snakeskin gourami (Trichopodus pectoralis), an economically important fish species native to Southeast Asia, and it was previously shown that it can infect and cause lethal streptococcosis in the fish. Results: In this study, we present the complete genome of S. suis 3112. Molecular sequence analysis revealed that it belongs to serotype 6, sequence type 2340. Phylogenetic analysis showed that the bacterium clustered with healthy-pig S. suis isolates, suggestive of an ultimate swine (as opposed to human) origin of the bacterium. Two fluoroquinolone resistance genes are present in the bacterial genome, namely patA and patB. Our results showed that both genes are expressed in our bacterium, and the bacterium is resistant to norfloxacin, but is still sensitive to other fluoroquinolones, including ciprofloxacin, enrofloxacin, and sparfloxacin. Additionally, the bacterium is sensitive to β-lactams, tetracyclines, sulphonamides, and an aminoglycoside. Conclusions: This study reports and describes the complete genome of S. suis 3112, the first isolate of S. suis known to infect fish, and provides further insights into the bacterial isolate, particularly regarding its drug resistance profile. These results will facilitate further investigations of the comparative genomics and pathogenic characteristics of S. suis, as well as the development of control strategies against this newly-identified fish pathogen. [ABSTRACT FROM AUTHOR]

Saprolegniosis, also called water mould, induces a cotton or wool‐like white growth on fish skin. It can kill fish at all stages of life, from eggs to adults. It is caused by oomycetes from the genus Saprolegnia and causes fish mortality and huge financial losses to fish farms and hatcheries. Saprolegnia species are endemic and ubiquitous in all freshwater habitats around the world. The exposing factors for saprolegniosis are still largely unknown, but stressors such as temperature shocks, poor water quality, handling and high fish density have been associated with outbreaks. For decades, malachite green was the most effective treatment against Saprolegnia infection, but it has been banned due to its carcinogenic and toxic effects. This has forced farmers to use alternative disinfection methods against Saprolegnia infection, such as hydrogen peroxide, formalin, Bronopol, NaCl, acetic acid and ozone, although many may have safety concerns or are impractical to use. This has led to the investigation of plant‐based compounds with antifungal and antibacterial properties against saprolegniosis. These include extracts of certain herbs, onion, garlic, extracts of the plant Chrysanthemum, essential oils of Eryngium campestre, Mentha piperita, Cuminum cyminum and Thymus linearis, which include a variety of phenolic compounds and fatty acids with antifungal properties. This review combines the current knowledge regarding the predisposing factors to Saprolegnia infections and current methods to prevent and treat them, including those under further research. Thus far, many compounds have been tested and studied, but an effective, suitable and safe compound to treat Saprolegnia infection remains to be found. [ABSTRACT FROM AUTHOR]

Piscirickettsiosis is the main cause of mortality in salmonids of commercial importance in Chile, which is caused by Piscirickettsia salmonis, a Gram‐negative, γ‐proteobacteria that can produce biofilm as one of its virulence factors. The Chilean salmon industry uses large amounts of antibiotics to control piscirickettsiosis outbreaks, which has raised concern about its environmental impact and the potential to induce antibiotic resistance. Thus, the use of phytogenic feed additives (PFA) with antibacterial activity emerges as an interesting alternative to antimicrobials. Our study describes the antimicrobial action of an Andrographis paniculate‐extracted PFA on P. salmonis planktonic growth and biofilm formation. We observed complete inhibition of planktonic and biofilm growth with 500 and 400 μg/mL of PFA for P. salmonis LF‐89 and EM‐90‐like strains, respectively. Furthermore, 500 μg/mL of PFA was bactericidal for both evaluated bacterial strains. Sub‐inhibitory doses of PFA increase the transcript levels of stress (groEL), biofilm (pslD), and efflux pump (acrB) genes for both P. salmonis strains in planktonic and sessile conditions. In conclusion, our results demonstrate the antibacterial effect of PFA against P. salmonis in vitro, highlighting the potential of PFA as an alternative to control Piscirickettsiosis. [ABSTRACT FROM AUTHOR]

Abstract Saprolegniosis, also called water mould, induces a cotton or wool‐like white growth on fish skin. It can kill fish at all stages of life, from eggs to adults. It is caused by oomycetes from the genus Saprolegnia and causes fish mortality and huge financial losses to fish farms and hatcheries. Saprolegnia species are endemic and ubiquitous in all freshwater habitats around the world. The exposing factors for saprolegniosis are still largely unknown, but stressors such as temperature shocks, poor water quality, handling and high fish density have been associated with outbreaks. For decades, malachite green was the most effective treatment against Saprolegnia infection, but it has been banned due to its carcinogenic and toxic effects. This has forced farmers to use alternative disinfection methods against Saprolegnia infection, such as hydrogen peroxide, formalin, Bronopol, NaCl, acetic acid and ozone, although many may have safety concerns or are impractical to use. This has led to the investigation of plant‐based compounds with antifungal and antibacterial properties against saprolegniosis. These include extracts of certain herbs, onion, garlic, extracts of the plant Chrysanthemum, essential oils of Eryngium campestre, Mentha piperita, Cuminum cyminum and Thymus linearis, which include a variety of phenolic compounds and fatty acids with antifungal properties. This review combines the current knowledge regarding the predisposing factors to Saprolegnia infections and current methods to prevent and treat them, including those under further research. Thus far, many compounds have been tested and studied, but an effective, suitable and safe compound to treat Saprolegnia infection remains to be found.

Abstract Aquaculture industries face challenges in fish production, primarily due to disease occurrence. Clinical manifestations of these diseases often indicate a considerable advancement in the pathological process. Infected fish often experience morbidity, leading to compromised welfare and substantial economic losses for production facilities. Therefore, prophylaxis and early detection of the pathogens could allow the facility managers to intervene or treat the disease before clinical disease signs occur. For the first time, environmental DNA/RNA (collectively eNA) was used to detect the salmonid pathogen Aeromonas salmonicida subsp. salmonicida in the water hosting rainbow trout (Oncorhynchus mykiss). This took place under controlled experimental conditions, before the occurrence of clinical outcome and mortality. The concentration of bacteria in the water was determined by using a standard curve. To determine the window of possible intervention, bacterial concentration was measured from an early stage of disease and throughout when fish became moribund. Furthermore, the acute phase protein, serum amyloid A produced by rainbow trout, was detected in the water and was found to be up‐regulated 4 days post challenge. It was thereby shown that it is possible to detect strong immune responses in the fish in a non‐invasive manner. The detection of eNA from the water can be used as a tool to detect pathogens in the early phase of infection before the disease becomes severe. This will facilitate interventions to limit the disease and increase the welfare of the fish, which is becoming a requirement for a more sustainable and ethical animal production.

ABSTRACT The incidence of Vibrio vulnificus infections, with high mortality rates in humans and aquatic animals, has escalated, highlighting a significant public health challenge. Currently, reliable markers to identify strains with high virulence potential are lacking, and the understanding of evolutionary drivers behind the emergence of pathogenic strains is limited. In this study, we analyzed the distribution of virulent genotypes and phenotypes to discern the infectious potential of V. vulnificus strains isolated from three distinct sources. Most isolates, traditionally classified as biotype 1, possessed the virulence-correlated gene-C type. Environmental isolates predominantly exhibited YJ-like alleles, while clinical and diseased fish isolates were significantly associated with the nanA gene and pathogenicity region XII. Hemolytic activity was primarily observed in the culture supernatants of clinical and diseased fish isolates. Genetic relationships, as determined by multiple-locus variable-number tandem repeat analysis, suggested that strains originating from the same source tended to cluster together. However, multilocus sequence typing revealed considerable genetic diversity across clusters and sources. A phylogenetic analysis using single nucleotide polymorphisms of diseased fish strains alongside publicly available genomes demonstrated a high degree of evolutionary relatedness within and across different isolation sources. Notably, our findings reveal no direct correlation between phylogenetic patterns, isolation sources, and virulence capabilities. This underscores the necessity for proactive risk management strategies to address pathogenic V. vulnificus strains emerging from environmental reservoirs.IMPORTANCEAs the global incidence of Vibrio vulnificus infections rises, impacting human health and marine aquacultures, understanding the pathogenicity of environmental strains remains critical yet underexplored. This study addresses this gap by evaluating the virulence potential and genetic relatedness of V. vulnificus strains, focusing on environmental origins. We conduct an extensive genotypic analysis and phenotypic assessment, including virulence testing in a wax moth model. Our findings aim to uncover genetic and evolutionary factors that drive pathogenic strain emergence in the environment. This research advances our ability to identify reliable virulence markers and understand the distribution of pathogenic strains, offering significant insights for public health and environmental risk management.

Yersinia ruckeri is the cause of hemorrhagic septicemia, known as enteric redmouth disease, in salmonid fish species. This bacterial pathogen can form biofilms on abiotic surfaces of aquaculture settings or even on the surfaces of the fish themselves, contributing to their persistence in the aquatic environment. Detection methods for this and other fish pathogens can be time-consuming and lack specificity and sensitivity, limiting timely monitoring, the treatment of microbial infections, and effective control of their transmission in aquaculture settings. Rapid and sensitive detection methods for nucleic acids can be crucial for an appropriate surveillance of bacterial pathogens, and the CRISPR/Cas-based assays have emerged as a good alternative since it has been proven to be a useful tool for the rapid, specific, and sensitive detection of viruses and some bacteria. In this study, we explored the capability of the CRISPR/Cas13a system (SHERLOCK) to specifically detect both DNA and RNA (gene transcripts) from planktonic and biofilm samples of the bacterial fish pathogen Y. ruckeri. The assay was designed to detect the gyrA gene and the small noncoding RNAs (sRNAs) MicA and RprA from planktonic cultures and biofilm samples prepared in marine broth. The specific crRNA designed for these gene targets included a 28 nt specific gene sequence, and a scaffold sequence necessary for Cas13-binding. For all the assays, the nucleic acids obtained from samples were previously subjected to isothermal amplification with the recombinase polymerase amplification (RPA) method and the subsequent T7 transcription of the RPA amplicons. Finally, the detection of nucleic acids of Y. ruckeri was by means of a reporter signal released by the Cas13a collateral RNA cleavage triggered upon target recognition, measured by fluorescence- or lateral-flow-based readouts. This CRISPR/Cas13a-based assay was able to specifically detect both DNA and sRNAs from the Y. ruckeri samples, and the sensitivity was comparable to that obtained with qPCR analysis, highlighting the potential applicability of this CRISPR/Cas13a-based assay for fish pathogen surveillance. [ABSTRACT FROM AUTHOR]

Edwardsiella piscicida is an important fish pathogen with a broad host that causes substantial economic losses in the aquaculture industry. Ferric uptake regulator (Fur) is a global transcriptional regulator and contains two typical domains, the DNA-binding domain and dimerization domain. In a previous study, we obtained a mutant strain of full-length fur of E. piscicida, TX01Δfur, which displayed increased siderophore production and stress resistance factors and decreased pathogenicity. To further reveal the regulatory mechanism of Fur, the DNA-binding domain (N-terminal) of Fur was knocked out in this study and the mutant was named TX01Δfur2. We found that TX01Δfur2 displayed increased siderophore production and enhanced adversity tolerance, including a low pH, manganese, and high temperature stress, which was consistent with the phenotype of TX01Δfur. Contrary to TX01Δfur, whose virulence was weakened, TX01Δfur2 displayed an ascended invasion of nonphagocytic cells and enhanced destruction of phagocytes via inducing overpowering or uncontrollable pyroptosis, which was confirmed by the fact that TX01Δfur2 induced higher levels of cytotoxicity, IL-1β, and p10 in macrophages than TX01. More importantly, TX01Δfur2 displayed an increased global virulence to the host, which was confirmed by the result that TX01Δfur2 caused higher lethality outcomes for healthy tilapias than TX01. These results demonstrate that the mutation of the Fur N-terminal domain augments the resistance level against the stress and pathogenicity of E. piscicida, which is not dependent on the bacterial number in host cells or host tissues, although the capabilities of biofilm formation and the motility of TX01Δfur2 decline. These interesting findings provide a new insight into the functional analysis of Fur concerning the regulation of virulence in E. piscicida and prompt us to explore the subtle regulation mechanism of Fur in the future. [ABSTRACT FROM AUTHOR]

Aquatic pathogens contribute to the most severe economic loss in fishes. Nanoparticles are being developed as potent antimicrobial agents against various pathogens. The bacteria Aeromonas caviae is one of the most common aquatic pathogens that infects the majority of economically important fishes causing significant losses to the aquaculture industry. This study involved synthesizing and characterizing of a novel chitosan‐selenium nanoflower employing multiple spectroscopic and microscopic approaches. The UV‐vis spectra obtained at 265 nm indicated the formation of the Chitosan‐selenium nanoflower. The particle size analysis revealed the size of the nanoflowers to be 186.3 nm. The transmission electron micrographs revealed a unique nanoflower-like morphology. XRD spectrum revealed amorphous nature. The Raman spectrum showed a strong resonance peak at 254 cm−1 which is a characteristic absorption band for monoclinic Se and α‐Se. Cytotoxicity analysis of the synthesized nanoflowers against isolated fish pathogen Aeromonas caviae showed increasing toxicity in a dose‐dependent manner with maximum cytotoxicity of 75.06% at 1000 µg/mL. The DCFDA assay was conducted to estimate the increase in reactive oxygen species (ROS) production, and the highest percentage increase in ROS of 96.02% was observed at 1000 µg/mL. The lipid peroxidation assay was performed by quantification of the lipid oxidation product malondialdehyde (MDA). The highest percentage of lipid peroxidation was found to occur at a dose of 500 µg/mL. As a result, the synthesized chitosan‐selenium nanoflowers can be exploited as a promising antibacterial treatment against the fish pathogen Aeromonas caviae. [ABSTRACT FROM AUTHOR]

Streptococcus iniae is a bacterial pathogen that causes streptococcosis, leading to significant losses in fish aquaculture globally. This study reported a newly developed probe‐based quantitative polymerase chain reaction (qPCR) method for the detection of S. iniae. The primers and probes were designed to target the lactate oxidase gene. The optimized method demonstrated a detection limit of 20 copies per reaction and was specific to S. iniae, as evidenced by no cross‐reactivity when assayed against genetic materials extracted from 23 known aquatic animal pathogens, and fish samples infected with Streptococcus agalactiae or Streptococcus dysgalactiae. To validate the newly developed qPCR protocol with field samples, fish specimens were systematically investigated following the Food and Agriculture Organization of the United Nations & Network of Aquaculture Centres in Asia‐Pacific three diagnostic levels approach, which integrated basic and advanced techniques for disease diagnosis, including observation of gross signs (level I), bacterial isolation (level II), qPCR and 16S rDNA sequencing (level III). The result showed that 7/7 affected farms (three Asian seabass farms and four tilapia farms) experiencing clinical signs of streptococcosis were diagnosed positive for S. iniae. qPCR assays using DNA extracted directly from fish tissue detected S. iniae in 11 out of 36 fish samples (30.6%), while 24 out of 36 samples (66.7%) tested positive after an enrichment step, including apparently healthy fish from affected farms. Bacterial isolation of S. iniae was only successful in a proportion of clinically diseased fish but not in healthy‐looking fish from the same farm. Overall, the newly developed qPCR protocol combined with enrichment would be a useful tool for the diagnosis and surveillance of S. iniae infections in fish populations, thereby aiding in the disease control and prevention. [ABSTRACT FROM AUTHOR]

Fish health can influence important population-level processes, especially those trying to deal with challenges posed by the natural environment, and growth, which in turn affect stock productivity. Two experiments were conducted to evaluate the metabolites of epidermal mucus in response to two threats of environment faced by the fishes: starvation and pathogenic attack; with the aim to check the alteration in values of innate immune parameters in the epidermal mucus of three Indian major carps, Catla catla, Labeo rohita, and Cirrhinus mrigala. Study showed that, in all three species the amount of mucus secretion and parameters like protein, carbohydrate and lipid of skin mucus altered after both the challenge trials and their concentration differed with respect to species. In particular, immune parameters including lysozyme, alkaline phosphatase and protease increased after both challenges (starvation and pathogenic attack) to face the stress. In contrast myeloperoxidase activity decreased significantly (P < 0.05) after the challenge of starvation but its value increased with pathogen challenge. Variations in mucosal parameters thus could be employed as possible non-invasive biomarkers for studying fish physiology in response to environmental stresses and as an integrated approach to assess the particular species response to the specific condition. [ABSTRACT FROM AUTHOR]

Chlamydiae like Chlamydia trachomatis and Chlamydia psittaci are well-known human and animal pathogens. Yet, the chlamydiae are a much larger group of evolutionary ancient obligate intracellular bacteria that includes predominantly symbionts of protists and diverse animals. This makes them ideal model organisms to study evolutionary transitions from symbionts in microbial eukaryotes to pathogens of humans. To this end, comparative genome analysis has served as an important tool. Genome sequence data for many chlamydial lineages are, however, still lacking, hampering our understanding of their evolutionary history. Here, we determined the first high-quality draft genome sequence of the fish pathogen " Candidatus Clavichlamydia salmonicola", representing a separate genus within the human and animal pathogenic Chlamydiaceae. The " Ca. Clavichlamydia salmonicola" genome harbors genes that so far have been exclusively found in Chlamydia species suggesting that basic mechanisms important for the interaction with chordate hosts have evolved stepwise in the history of chlamydiae. Thus, the genome sequence of " Ca. Clavichlamydia salmonicola" allows to constrain candidate genes to further understand the evolution of chlamydial virulence mechanisms required to infect mammals. [ABSTRACT FROM AUTHOR]

The fermented broth of lactic acid bacteria (LFB)was the side product while lactic acid bacteria produced; however, it was abundant of antimicrobial activity. That might be a good candidate for feed additive in aquaculture. In this study, the fermented broth of Lactobacillus paracasei GL-156, the most antibacterial activity, was further evaluated the safety, growth performance gut microbiota. Common carp consumed the feed in dose of 1000 ppm LFB, the survival was no significant different with control group, but the growth performance was significant increased. In the intestine microbiota analysis, the treatment fish shown the decrease of vibrio, but no different of total bacterial count. These results demonstrate the LFB from GL-156 in enhancing the growth performance, inhibiting the growth of pathogenic bacteria, and promoting the growth of probiotic organisms. Compare with live probiotics currently used in aquaculture, using LFB will achieved beneficial effects equally, while providing superior tolerance to manufacturing processes, and in the lower cost. [ABSTRACT FROM AUTHOR]

Summary: It has been clarified that pathogens bind to glycosphingolipid (GSL) receptors in mammals, but there have been very few reports on pathogen-binding GSLs in fish. Vibrios are facultative anaerobic bacteria ubiquitous in marine and brackish environments. They are members of the normal intestinal microflora of healthy fish, but some species can cause a disease called vibriosis in fish and shellfish when the hosts are physiologically or immunologically weakened. The adherence of vibrios to host intestinal tracts is a significant event not only for survival and growth but also in terms of pathogenicity. We show in this mini-review that sialic acid-containing GSLs (gangliosides), GM4 and GM3, are receptors to which vibrios adhere to epithelial cells in the intestinal tract of fish. We also describe the enzymes responsible for synthesizing these Vibrio-binding gangliosides in fish. [ABSTRACT FROM AUTHOR]

Renibacterium salmoninarum (Rs) is the etiological agent of bacterial kidney disease (BKD), which significantly affects farmed and wild salmonids worldwide. Although the whole genome of Rs (~3.1 million nucleotides) is highly conserved, genomic epidemiology analyses have identified four sub‐lineages from Chilean isolates. A total of 94 Rs genomes from the BIGSdb aquaculture database were aligned and compared using bioinformatics tools, identifying 2199 independent single‐nucleotide polymorphisms (SNPs) spread along the genome. A detailed analysis of the distribution of the SNPs showed five local zones of a length in the range of 10–15 kbp that should be used to unambiguously identify a specific sub‐lineage. Based on the Rs type strain DSM 20767T, we designed multiplex PCR primers that produce specific amplification products which were further sequenced by the Sanger method to obtain the genotype of the sub‐lineage. For the genetic typing, we evaluated 27 Rs isolates recovered from BKD outbreaks from different fish species and regions of Chile. Based on the findings reported here, we propose the PCR approach as a valuable tool for the rapid and reliable studying of the relationships between Rs isolates and the different sub‐lineages without requiring the sequencing of the entire genome. [ABSTRACT FROM AUTHOR]

Recently, fabricated nanoparticles (NPs), which can efficiently penetrate biological systems, have found increased usage in the health and hygiene industries. Microbial enzymes and proteins have recently shown their potential to act as reducing agents for the production of NPs, thereby providing an alternative to physical and chemical methods. Not only is this approach efficient and cost-effective, but it also produces a minimal ecological footprint. In this study, zinc oxide nanoparticles (ZnO NPs) were synthesized using probiotic bacteria (Lactobacillus fermentum) as the reducing and capping agent. Several analytical methods, including Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), ultraviolet–visible spectroscopy (UV–Vis), and atomic force microscopy (AFM), were used to analyze the produced ZnO NPs. The SEM analysis confirmed the spherical form of the nanoparticles and estimated their average size to be between 100 and 120 nm. FT-IR analysis verified that the ZnO NPs' surfaces contained many functional groups. X-ray diffraction examination evidenced that the biogenically produced nanoparticles were crystalline. AFM analysis revealed that the nanoparticles' size was about 90–100 nm. The maximum absorption peak, determined via a UV–visible spectrophotometer, was 510 nm. The synthesized ZnO NPs' antimicrobial activity against various bacterial strains was tested, and the highest level of antimicrobial activity was noted against a Vibrio harveyi strain. The maximum concentration, namely, 20 mM of ZnO NPs, showed the highest antimicrobial activity. These observations indicate that the synthesized ZnO NPs possess remarkable antimicrobial potency. This method is an efficient, environmentally friendly, cost-effective approach for producing ZnO NPs that are useful for various biomedical applications. [ABSTRACT FROM AUTHOR]

Yersinia ruckeri is the cause of hemorrhagic septicemia, known as enteric redmouth disease, in salmonid fish species. This bacterial pathogen can form biofilms on abiotic surfaces of aquaculture settings or even on the surfaces of the fish themselves, contributing to their persistence in the aquatic environment. Detection methods for this and other fish pathogens can be time-consuming and lack specificity and sensitivity, limiting timely monitoring, the treatment of microbial infections, and effective control of their transmission in aquaculture settings. Rapid and sensitive detection methods for nucleic acids can be crucial for an appropriate surveillance of bacterial pathogens, and the CRISPR/Cas-based assays have emerged as a good alternative since it has been proven to be a useful tool for the rapid, specific, and sensitive detection of viruses and some bacteria. In this study, we explored the capability of the CRISPR/Cas13a system (SHERLOCK) to specifically detect both DNA and RNA (gene transcripts) from planktonic and biofilm samples of the bacterial fish pathogen Y. ruckeri. The assay was designed to detect the gyrA gene and the small noncoding RNAs (sRNAs) MicA and RprA from planktonic cultures and biofilm samples prepared in marine broth. The specific crRNA designed for these gene targets included a 28 nt specific gene sequence, and a scaffold sequence necessary for Cas13-binding. For all the assays, the nucleic acids obtained from samples were previously subjected to isothermal amplification with the recombinase polymerase amplification (RPA) method and the subsequent T7 transcription of the RPA amplicons. Finally, the detection of nucleic acids of Y. ruckeri was by means of a reporter signal released by the Cas13a collateral RNA cleavage triggered upon target recognition, measured by fluorescence- or lateral-flow-based readouts. This CRISPR/Cas13a-based assay was able to specifically detect both DNA and sRNAs from the Y. ruckeri samples, and the sensitivity was comparable to that obtained with qPCR analysis, highlighting the potential applicability of this CRISPR/Cas13a-based assay for fish pathogen surveillance.

Flavobacterium columnare causes columnaris disease in freshwater fish in both natural and aquaculture settings. This disease is often lethal, especially when fish population density is high, and control options such as vaccines are limited. The type IX secretion system (T9SS) is required for F. columnare virulence, but secreted virulence factors have not been fully identified. Many T9SS-secreted proteins are predicted peptidases, and peptidases are common virulence factors of other pathogens. T9SS-deficient mutants, such as ΔgldN and ΔporV, exhibit strong defects in secreted proteolytic activity. The F. columnare genome has many peptidase-encoding genes that may be involved in nutrient acquisition and/or virulence. Mutants lacking individual peptidase-encoding genes, or lacking up to ten peptidase-encoding genes, were constructed and examined for extracellular proteolytic activity, for growth defects, and for virulence in zebrafish and rainbow trout. Most of the mutants retained virulence, but a mutant lacking 10 peptidases, and a mutant lacking the single peptidase TspA exhibited decreased virulence in rainbow trout fry, suggesting that peptidases contribute to F. columnare virulence. [ABSTRACT FROM AUTHOR]

Most of the emerging diseases that threaten humans are caused by RNA viruses which are extremely mutable during evolution. The fish RNA virus, viral hemorrhagic septicemia virus (VHSV) can infect a broad range of aquatic animal hosts, but the transmissibility of VHSV to mammals has not been thoroughly investigated. Therefore, our study aimed to investigate the potential adverse effects of VHSV in mammals. Briefly, the survival of VHSV was determined using only minimum essential media (MEM-2) and mammalian SNU-1411 and hepa-1c1c7s cells at 15°C and 37°C. Mice (Mus musculus, 27.3 ± 1.9 g) were intravenously injected with VHSV (2.37E+05 TCID50 · mice.–1) in triplicate. Clinical signs and survival rates were examined at 14 days post-challenge, and infection was confirmed in the surviving mice. The 50% tissue culture infective dose (TCID50) and polymerase chain reaction analysis were used to determine viral titers and the infection rate, respectively. The titer of VHSV suspended in MEM-2 at 15°C was reduced by only one log after 8 days, whereas the virus maintained at 37°C was inactivated 8 days post-inoculation (dpi). There were no recognizable cytopathic effects in either SNU-1411 or hepa-1c1c7s cells inoculated with VHSV at 15°C and 37°CVHSV in those cell lines at 37°C was rapidly decreased and eventually inactivated at 12 dpi, whereas virus at 15°C remained at low concentrations without replication. In vivo experiment showed that there were no signs of disease, mortality, or infection in VHSV-infected mice. The results of this study indicate that it is highly unlikely that VHSV can infect mammals including humans.

Edwardsiella piscicida is an important fish pathogen with a broad host that causes substantial economic losses in the aquaculture industry. Ferric uptake regulator (Fur) is a global transcriptional regulator and contains two typical domains, the DNA-binding domain and dimerization domain. In a previous study, we obtained a mutant strain of full-length fur of E. piscicida, TX01Δfur, which displayed increased siderophore production and stress resistance factors and decreased pathogenicity. To further reveal the regulatory mechanism of Fur, the DNA-binding domain (N-terminal) of Fur was knocked out in this study and the mutant was named TX01Δfur2. We found that TX01Δfur2 displayed increased siderophore production and enhanced adversity tolerance, including a low pH, manganese, and high temperature stress, which was consistent with the phenotype of TX01Δfur. Contrary to TX01Δfur, whose virulence was weakened, TX01Δfur2 displayed an ascended invasion of nonphagocytic cells and enhanced destruction of phagocytes via inducing overpowering or uncontrollable pyroptosis, which was confirmed by the fact that TX01Δfur2 induced higher levels of cytotoxicity, IL-1β, and p10 in macrophages than TX01. More importantly, TX01Δfur2 displayed an increased global virulence to the host, which was confirmed by the result that TX01Δfur2 caused higher lethality outcomes for healthy tilapias than TX01. These results demonstrate that the mutation of the Fur N-terminal domain augments the resistance level against the stress and pathogenicity of E. piscicida, which is not dependent on the bacterial number in host cells or host tissues, although the capabilities of biofilm formation and the motility of TX01Δfur2 decline. These interesting findings provide a new insight into the functional analysis of Fur concerning the regulation of virulence in E. piscicida and prompt us to explore the subtle regulation mechanism of Fur in the future.

Flavobacterium columnare causes columnaris disease in freshwater fish in both natural and aquaculture settings. This disease is often lethal, especially when fish population density is high, and control options such as vaccines are limited. The type IX secretion system (T9SS) is required for F. columnare virulence, but secreted virulence factors have not been fully identified. Many T9SS-secreted proteins are predicted peptidases, and peptidases are common virulence factors of other pathogens. T9SS-deficient mutants, such as ΔgldN and ΔporV, exhibit strong defects in secreted proteolytic activity. The F. columnare genome has many peptidase-encoding genes that may be involved in nutrient acquisition and/or virulence. Mutants lacking individual peptidase-encoding genes, or lacking up to ten peptidase-encoding genes, were constructed and examined for extracellular proteolytic activity, for growth defects, and for virulence in zebrafish and rainbow trout. Most of the mutants retained virulence, but a mutant lacking 10 peptidases, and a mutant lacking the single peptidase TspA exhibited decreased virulence in rainbow trout fry, suggesting that peptidases contribute to F. columnare virulence.

Pseudomonas species are metabolically versatile bacteria able to exploit a wide range of ecological niches. Different Pseudomonas species can grow as free-living cells, biofilms, or associated with plants or animals, including humans, and their ecological success partially lies in their ability to grow and adapt to different temperatures. These bacteria are relevant for human activities, due to their clinical importance and their biotechnological potential for different applications such as bioremediation and the production of biopolymers, surfactants, secondary metabolites, and enzymes. In agriculture, some of them can act as plant growth promoters and are thus used as inoculants, whereas others, like P. syringae pathovars, can cause disease in commercial crops. This review aims to provide an overview of the temperature-response mechanisms in Pseudomonas species, looking for novel features or strategies based on techniques such as transcriptomics and proteomics. We focused on temperature-dependent traits mainly associated with virulence, host colonization, survival, and production of secondary metabolites. We analyzed human, animal, and plant pathogens and plant growth-promoting Pseudomonas species, including P. aeruginosa, P. plecoglossicida, several P. syringae pathovars, and P. protegens. Our aim was to provide a comprehensive view of the relevance of temperature-response traits in human and animal health and agricultural applications. Our analysis showed that features relevant to the bacterial-host interaction are adjusted to the environmental or host temperature regardless of the optimal growth temperature in the laboratory, and thus contribute to improving bacterial fitness. Key points: • In Pseudomonas species, temperature impacts the bacterial-host interaction. • Interaction traits are expressed at temperatures different from the optimal reported. • The bacterial-host interaction could be affected by climate change. [ABSTRACT FROM AUTHOR]

Saprolegnia parasitica induces heavy mortality in aquaculture. The detection of S. parasitica is often time consuming and uncertain, making it difficult to manage the disease. We validated a previously published real-time quantitative PCR (qPCR) assay to confirm the presence of S. parasitica in fish and in water using environmental DNA (eDNA) quantification. Analytical sensitivity and specificity of the assay was assessed in silico, in vitro and the qPCR assay was compared with microbiological cultivation methods to detect and quantify S. parasitica in water samples from a controlled fish exposure experiment and from fish farms. Furthermore, we compared the use of an agar cultivation method and the qPCR assay to detect S. parasitica directly from mucus samples taken from the fish surface. The analytical sensitivity and specificity of the qPCR assay were high. The qPCR assay detected 100% of S. parasitica-positive water samples. In a field study, the qPCR assay and a microwell plate (MWP) enumeration method correlated significantly. Furthermore, the qPCR assay could be used to confirm the presence of S. parasitica in skin mucus. Thus, the qPCR assay could complement diagnostic methods in specifically detecting saprolegniosis in fish and used as a surveillance method for S. parasitica pathogen in aquaculture environments. [ABSTRACT FROM AUTHOR]

Flavobacterium columnare causes columnaris disease in wild and aquaculture-reared freshwater fish. F. columnare virulence mechanisms are not well understood, and current methods to control columnaris disease are inadequate. Iron acquisition from the host is important for the pathogenicity and virulence of many bacterial pathogens. F. columnare iron acquisition has not been studied in detail. We identified genes predicted to function in siderophore production for ferric iron uptake. Genes predicted to encode the proteins needed for siderophore synthesis, export, uptake, and regulation were deleted from F. columnare strain MS-FC-4. The mutants were examined for defects in siderophore production, for growth defects in iron-limited conditions, and for virulence against zebrafish and rainbow trout. Mutants lacking all siderophore activity were obtained. These mutants displayed growth defects when cultured under iron-limited conditions, but they retained virulence against zebrafish and rainbow trout similar to that exhibited by the wild type, indicating that the F. columnare MS-FC-4 siderophores are not required for virulence under the conditions tested. [ABSTRACT FROM AUTHOR]

This study aims to determine the antimicrobial resistance (AMR) pattern in freshwater ornamental cyprinids, such as Goldfish and Tiger barb. Molecular characterization of bacterial isolates confirmed the presence of 7 bacterial isolates in Goldfish and 6 in Tiger barb. Antimicrobial susceptibility test using 36 antibiotics revealed a higher resistance pattern for bacitracin, rifampicin, trimethoprim, cefalexin, ampicillin, amoxicillin, nalidixic acid and nitrofurantoin. Sulphafurazole, norfloxacin and ciprofloxacin were effective against all the bacterial isolates derived from Goldfish and Tiger barb. Most bacterial isolates exhibited > 0.2 multi-drug resistance index (MDR), indicating the severity of antibiotic use in the culture system. The finding of the present study suggests that ornamental fish may act as the reservoir of MDR bacteria and dissemination of resistance genes to clinical and human commensal bacteria through horizontal gene transfer. [ABSTRACT FROM AUTHOR]

Similar to other marine holobionts, fish are colonized by complex microbial communities that promote their health and growth. Fish-associated microbiota is emerging as a promising source of bioactive metabolites. Pleuronectes platessa (European plaice, plaice), a flatfish with commercial importance, is common in the Baltic Sea. Here we used a culture-dependent survey followed by molecular identification to identify microbiota associated with the gills and the gastrointestinal tract (GIT) of P. platessa, then profiled their antimicrobial activity and metabolome. Altogether, 66 strains (59 bacteria and 7 fungi) were isolated, with Proteobacteria being the most abundant phylum. Gill-associated microbiota accounted for higher number of isolates and was dominated by the Proteobacteria (family Moraxellaceae) and Actinobacteria (family Nocardiaceae), whereas Gram-negative bacterial families Vibrionaceae and Shewanellaceae represented the largest group associated with the GIT. The EtOAc extracts of the solid and liquid media cultures of 21 bacteria and 2 fungi representing the diversity of cultivable plaice-associated microbiota was profiled for their antimicrobial activity against three fish pathogens, human bacterial pathogen panel (ESKAPE) and two human fungal pathogens. More than half of all tested microorganisms, particularly those originating from the GIT epithelium, exhibited antagonistic effect against fish pathogens (Lactococcus garvieae, Vibrio ichthyoenteri) and/or human pathogens (Enterococcus faecium, methicillin-resistant Staphylococcus aureus). Proteobacteria represented the most active isolates. Notably, the solid media extracts displayed higher activity against fish pathogens, while liquid culture extracts were more active against human pathogens. Untargeted metabolomics approach using feature-based molecular networking showed the high chemical diversity of the liquid extracts that contained undescribed clusters. This study highlights plaice-associated microbiota as a potential source of antimicrobials for the control of human and the aquaculture-associated infections. This is the first study reporting diversity, bioactivity and chemical profile of culture-dependent microbiota of plaice. [ABSTRACT FROM AUTHOR]

Renibacterium salmoninarum, a Gram‐positive intracellular pathogen, is the causative agent of bacterial kidney disease (BKD), the impacts of which are high mortalities and economic losses for the salmon industry. This study provides novel analyses for the whole‐genome sequences of 50 R. salmoninarum isolates and the reference strain ATCC 33209 using a pan‐genomic approach to elucidate phylogenomic relationships and identify unique and shared genes associated with pathogenicity and infection mechanisms. Genome size varied from 3,061,638 to 3,155,332 bp; gene count from 3452 to 3580; and predicted coding sequences from 3402 to 3527. Comparative analyses revealed an open, but approaching closed, pan‐genome. The pan‐genome analysis recovered 4064 genes, with a core genome containing 3306 genes. Phylogenetic analysis of R. salmoninarum showed high genomic homogeneity, apart from one isolate obtained from Salmo trutta in Norway. All genomes presented the 57‐kDa protein (p57). Strain ATCC 33209 and the Chilean isolates H‐2 and DJ2R presented two copies of the msa gene, while the remaining isolates had one copy. The pan‐genome analysis further identified differences in the number of copies and length of the signalling peptide for p57, the principal virulence factor reported for this bacterium. This heterogeneity could be associated with the secretion levels of p57, potentially influencing virulence. Additionally identified were numerous common genes related to iron uptake, the stress response and regulation, and cell signalling—all of which constitute the pathogenic repertoire of R. salmoninarum. This investigation provides information that is applicable in future studies for identifying therapeutic targets and/or for designing new strategies (e.g., vaccines) to prevent BKD infections in salmon farming. [ABSTRACT FROM AUTHOR]

Abstract Objective The objective of this research were to screen quorum quenching activity compound from phyllosphere bacteria as well as antibiofilm activity against several fish pathogen bacteria such as Aeromonas hydrophila, Streptococcus agalactiae, and Vibrio harveyi. Results We found eight phyllosphere bacteria isolates with potential quorum quenching activity to inhibit Chromobacterium violaceum as indicator bacteria. Crude extracts (20 mg/mL) showed various antibiofilm activity against fish pathogenic bacteria used in this study. Isolate JB 17B showed the highest activity to inhibit biofilm formation of A. hydrophila and V. harveyi, meanwhile isolate JB 3B showed the highest activity to inhibit biofilm of S. agalactiae. From destruction assay, isolate JB 8F showed the highest activity to disrupt biofilm of A. hydrophila isolate JB 20B showed the highest activity to disrupt biofilm of V. harveyi, isolate JB 17B also showed the highest activity to disrupt biofilm of S. agalactiae.

Seven previously undescribed compounds, including four diketomorpholine alkaloids (1 ‒ 4), one indole diketopiperazine alkaloid (9), one chromone (10), and one benzoic acid derivative (13), and nine known compounds (5 – 8 , 11 , 12 , and 14 – 16) were isolated from two different fungal sources. Nine of these metabolites (1 – 9) were obtained from a seagrass-derived Aspergillus alabamensis SYSU-6778, while the others were obtained from a mixed culture of A. alabamensis SYSU-6778 and a co-isolated fungus A. fumigatiaffinis SYSU-6786. The chemical structures of the compounds were deduced via spectroscopic techniques (including HRESIMS, 1D and 2D NMR), chemical reactions, and ECD calculations. It is worth noting that compound 10 was identified as a defensive secondary metabolite of strain SYSU-6786, produced through the induction of compound 8 under co-culture conditions. Compounds 3 and 4 possessed a naturally rare isotryptophan core. Moreover, compounds 1 and 2 exhibited potent inhibitory activities against fish pathogenic bacterium Edwardsiella ictalurid, with minimum inhibitory concentration values of 10.0 μg/mL for both compounds. [Display omitted] • Sixteen metabolites (1 – 16) were purified from two seagrass-derived Aspergillus strains. • Compounds 10 and 13 were induced by compound 8 under co-culture conditions. • Compound 10 was identified as defensive specialized metabolite of A. fumigatiaffinis. • Compounds 1 and 2 exhibited antibacterial activity against fish pathogens. [ABSTRACT FROM AUTHOR]

Edwardsiella piscicida is an acute marine pathogen that causes severe damage to the aquaculture industry worldwide. The pathogenesis of E. piscicida is dependent mainly on the type III secretion system (T3SS) and type VI secretion system (T6SS), both of which are critically regulated by EsrB and EsrC. In this study, we revealed that fatty acids influence T3SS expression. Unsaturated fatty acids (UFAs), but not saturated fatty acids (SFAs), directly interact with EsrC, which abolishes the function of EsrC and results in the turn-off of T3/T6SS. Moreover, during the in vivo colonization of E. piscicida , host fatty acids were observed to be transported into E. piscicida through FadL and to modulate the expression of T3/T6SS. Furthermore, the esrC R38G mutant blocked the interaction between EsrC and UFAs, leading to dramatic growth defects in DMEM and impaired colonization in HeLa cells and zebrafish. In conclusion, this study revealed that the interaction between UFAs and EsrC to turn off T3/T6SS expression is essential for E. piscicida infection. [ABSTRACT FROM AUTHOR]

Recently, fabricated nanoparticles (NPs), which can efficiently penetrate biological systems, have found increased usage in the health and hygiene industries. Microbial enzymes and proteins have recently shown their potential to act as reducing agents for the production of NPs, thereby providing an alternative to physical and chemical methods. Not only is this approach efficient and cost-effective, but it also produces a minimal ecological footprint. In this study, zinc oxide nanoparticles (ZnO NPs) were synthesized using probiotic bacteria (Lactobacillus fermentum) as the reducing and capping agent. Several analytical methods, including Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), ultraviolet–visible spectroscopy (UV–Vis), and atomic force microscopy (AFM), were used to analyze the produced ZnO NPs. The SEM analysis confirmed the spherical form of the nanoparticles and estimated their average size to be between 100 and 120 nm. FT-IR analysis verified that the ZnO NPs’ surfaces contained many functional groups. X-ray diffraction examination evidenced that the biogenically produced nanoparticles were crystalline. AFM analysis revealed that the nanoparticles’ size was about 90–100 nm. The maximum absorption peak, determined via a UV–visible spectrophotometer, was 510 nm. The synthesized ZnO NPs’ antimicrobial activity against various bacterial strains was tested, and the highest level of antimicrobial activity was noted against a Vibrio harveyi strain. The maximum concentration, namely, 20 mM of ZnO NPs, showed the highest antimicrobial activity. These observations indicate that the synthesized ZnO NPs possess remarkable antimicrobial potency. This method is an efficient, environmentally friendly, cost-effective approach for producing ZnO NPs that are useful for various biomedical applications.

Tenacibaculosis caused by Tenacibaculum dicentrarchi is the third most important bacterial fish infection affecting the Chilean salmon industry. Losses to this disease are most frequently controlled by treatments with florfenicol and oxytetracycline. However, recent tenacibaculosis outbreaks were controlled through the extra‐label, oral administration of tiamulin, resulting in high treatment efficiency. In this study, we present an analysis of susceptibility patterns of 32 T. dicentrarchi isolates and the type strain CECT 7612T to tiamulin by determining the minimum inhibitory concentrations (MICs) according to the procedures recommended by the Clinical and Laboratory Standard Institute, but fixing incubation temperature to the more appropriate for the growth of T. dicentrarchi (18 ºC). The MICs of the T. dicentrarchi isolates were unimodally distributed (0.06–1.0 µg/ml range), while the CECT 7612T strain presented an MIC of 0.5 μg/ml. Calculations using Normalized Resistance Interpretation provided epidemiological cut‐off values of ≤1.0 µg/ml, with the 33 T. dicentrarchi classified as wild type. In Chile, tiamulin is authorized for use in other livestock species, but application in salmonids is extra‐label. Our presented in vitro results suggest that tiamulin is a viable alternative to florfenicol, specifically as tiamulin requires comparatively lower concentrations to inhibit T. dicentrarchi. Considering that tiamulin is also exclusively for veterinary use, is classified as "least important" by the World Health Organization and has not resulted in the development of bacterial resistance, pharmaceutical companies should be requested to register tiamulin and provide alternative antimicrobial treatments for the salmonid industry. [ABSTRACT FROM AUTHOR]

Reducing fish pathogenic bacteria outbreak is important as it results in economic losses in aquaculture. Antibiotics are an inevitable tool to control the pathogenic bacteria outbreak. However, the authorities are taking more regulations to prevent the emergence of drug-resistant strain development. Alternative materials to be employed for the purpose are needed from natural sources. In this research, we explored the antibacterial activity of natural products derived from hop against fish pathogenic strains Streptococcus iniae and Vibrio vulnificus. Antimicrobial activities of nine separates from hops were tested against S. iniae and V. vulnificus and identified. Xanthohumol (1) showed strongest antibacterial property against S. iniae. In addition, desmethylxanthohumol (4) and 8-prenylnaringenin (5) showed antibacterial against both of the tested pathogen strains. Antibacterial compounds were all prenylated flavonoids, and these might be used as index components for development of feed additives for fish in aquaculture. [ABSTRACT FROM AUTHOR]

Renibacterium salmoninarum, a slow‐growing facultative intracellular pathogen, is the causative agent of bacterial kidney disease, a chronic, progressive and granulomatous infection that threatens farmed and wild salmonids worldwide. Pathogenic R. salmoninarum colonizes tissues and invades the host through cell surface‐associated and secreted proteins. While correlations between iron acquisition genes and virulence have been demonstrated in vitro, these mechanisms have not undergone proteomic characterization. The present study applied a proteomic approach to elucidate the differences between the virulent Chilean R. salmoninarum H‐2 strain and the type strain ATCC 33209T. Analyses were conducted under normal (control) and iron‐limited conditions (DIP) emulating the host environment. Interestingly, strain H‐2 apparently responded better to the iron‐limited condition—for example, only this strain presented a significantly enriched iron ion homeostasis pathway. Furthermore, key virulence factors related to an iron‐limited environment were more abundant in strain H‐2. Importantly, the lack of iron favoured the expression of the 57‐kDa protein in strain H‐2, the principal virulence factor for R. salmoninarum. Our findings can be employed in the design and development of treatments targeted to iron uptake mechanisms (e.g. siderophore synthesis or haem uptake), which represents a promising therapeutic approach for treating this persistent fastidious bacterium. [ABSTRACT FROM AUTHOR]

Flavobacterium columnare causes columnaris disease in wild and cultured freshwater fish and is a major problem for sustainable aquaculture worldwide. The F. columnare type IX secretion system (T9SS) secretes many proteins and is required for virulence. The T9SS component GldN is required for secretion and gliding motility over surfaces. Genetic manipulation of F. columnare is inefficient, which has impeded identification of secreted proteins that are critical for virulence. Here, we identified a virulent wild-type F. columnare strain (MS-FC-4) that is highly amenable to genetic manipulation. This facilitated isolation and characterization of two deletion mutants lacking core components of the T9SS. Deletion of gldN disrupted protein secretion and gliding motility and eliminated virulence in zebrafish and rainbow trout. Deletion of porV disrupted secretion and virulence but not motility. Both mutants exhibited decreased extracellular proteolytic, hemolytic, and chondroitin sulfate lyase activities. They also exhibited decreased biofilm formation and decreased attachment to fish fins and other surfaces. Using genomic and proteomic approaches, we identified proteins secreted by the T9SS. We deleted 10 genes encoding secreted proteins and characterized the virulence of mutants lacking individual or multiple secreted proteins. A mutant lacking two genes encoding predicted peptidases exhibited reduced virulence in rainbow trout, and mutants lacking a predicted cytolysin showed reduced virulence in zebrafish and rainbow trout. The results establish F. columnare strain MS-FC-4 as a genetically amenable model to identify virulence factors. This may aid development of measures to control columnaris disease and impact fish health and sustainable aquaculture. IMPORTANCE Flavobacterium columnare causes columnaris disease in wild and aquaculture-reared freshwater fish and is a major problem for aquaculture. Little is known regarding the virulence factors involved in this disease, and control measures are inadequate. The type IX secretion system (T9SS) secretes many proteins and is required for virulence, but the secreted virulence factors are not known. We identified a strain of F. columnare (MS-FC-4) that is well suited for genetic manipulation. The components of the T9SS and the proteins secreted by this system were identified. Deletion of core T9SS genes eliminated virulence. Genes encoding 10 secreted proteins were deleted. Deletion of two peptidase-encoding genes resulted in decreased virulence in rainbow trout, and deletion of a cytolysin-encoding gene resulted in decreased virulence in rainbow trout and zebrafish. Secreted peptidases and cytolysins are likely virulence factors and are targets for the development of control measures. [ABSTRACT FROM AUTHOR]

Objective: Preliminary aim of this research work focuses on testing the efficacy of silver nanoparticles synthesised using water soluble polysaccharides extracted from seaweeds against pathogenic fish bacteria. The emergence of various virulence pathogens during the culturing period leads to mass mortality and influence the loss of aquaculture production. Methods: The intervention of nanotechnology in the field of aquaculture improves the growth and immunity of aquatic animals when the part of nanomaterials mixed with fish feed. Sometimes the green synthesis of silver nanoparticles prepared from the seaweed polysaccharide play a vital role in the disease controlling strategies in aquaculture farming. The nanoparticle formation and confirmation was charecterized by Ultra Visible Spectroscopy (UV–Vis), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope (SEM), Dynamic Light Scattering (DLS). Results and Conclusion: Biologically synthesized silver nanoparticles have been used as an alternative method for commercial antibiotics to control such pathogenic infection with 70–80% of survival rate. Polysaccharides of Caulerpa racemosa was used as a reducing agent in this present study. Particularly the phytochemicals are involved in the silver salt reduction process. Total polyphenol, total flavonoids, total antioxidant, hydroxyl radical scavenging assay and DPPH was found to be 12.32 mg/g, 18.44 mg/g, 38.12 mg/g equivalence, 82.2% and 49.05% respectively. Biologically synthesised AgNps capable of preventing the Pseudomonas aeruginosa infection in tilapia, it was proved by in vitro and in vivo antibacterial activities. Present study provided the ample achievement and these findings found to be effective in controlling the Pseudomonas infection in tilapia fishes.

The incidence of Vibrio vulnificus infections, with high mortality rates in humans and aquatic animals, has escalated, highlighting a significant public health challenge. Currently, reliable markers to identify strains with high virulence potential are lacking, and the understanding of evolutionary drivers behind the emergence of pathogenic strains is limited. In this study, we analyzed the distribution of virulent genotypes and phenotypes to discern the infectious potential of V. vulnificus strains isolated from three distinct sources. Most isolates, traditionally classified as biotype 1, possessed the virulence-correlated gene-C type. Environmental isolates predominantly exhibited YJ-like alleles, while clinical and diseased fish isolates were significantly associated with the nanA gene and pathogenicity region XII. Hemolytic activity was primarily observed in the culture supernatants of clinical and diseased fish isolates. Genetic relationships, as determined by multiple-locus variable-number tandem repeat analysis, suggested that strains originating from the same source tended to cluster together. However, multilocus sequence typing revealed considerable genetic diversity across clusters and sources. A phylogenetic analysis using single nucleotide polymorphisms of diseased fish strains alongside publicly available genomes demonstrated a high degree of evolutionary relatedness within and across different isolation sources. Notably, our findings reveal no direct correlation between phylogenetic patterns, isolation sources, and virulence capabilities. This underscores the necessity for proactive risk management strategies to address pathogenic V. vulnificus strains emerging from environmental reservoirs.IMPORTANCEAs the global incidence of Vibrio vulnificus infections rises, impacting human health and marine aquacultures, understanding the pathogenicity of environmental strains remains critical yet underexplored. This study addresses this gap by evaluating the virulence potential and genetic relatedness of V. vulnificus strains, focusing on environmental origins. We conduct an extensive genotypic analysis and phenotypic assessment, including virulence testing in a wax moth model. Our findings aim to uncover genetic and evolutionary factors that drive pathogenic strain emergence in the environment. This research advances our ability to identify reliable virulence markers and understand the distribution of pathogenic strains, offering significant insights for public health and environmental risk management., Competing Interests: The authors declare no conflict of interest.

Bacterial fish pathogens are one of the key challenges in the aquaculture industry, one of the fast-growing industries worldwide. These pathogens rely on arsenal of virulence factors such as toxins, adhesins, effectors and enzymes to promote colonization and infection. Translocation of virulence factors across the membrane to either the extracellular environment or directly into the host cells is performed by single or multiple dedicated secretion systems. These secretion systems are often key to the infection process. They can range from simple single-protein systems to complex injection needles made from dozens of subunits. Here, we review the different types of secretion systems in Gram-negative bacterial fish pathogens and describe their putative roles in pathogenicity. We find that the available information is fragmented and often descriptive, and hope that our overview will help researchers to more systematically learn from the similarities and differences between the virulence factors and secretion systems of the fish-pathogenic species described here. [ABSTRACT FROM AUTHOR]

In the current study, five Lactobacillus gasseri MA strains from human breast milk were investigated for their usage potentials in aquaculture and seafood products as probiotic and bio-protective. The strains were tested for their susceptibilities to various clinical antibiotics used in fish cultivation, antimicrobial activities against seven fish pathogens, alpha-amylase enzyme activities, and susceptibilities to sodium benzoate which used as a food preservative. The strains were susceptible to Ampicillin, Amoxicillin and Erythromycin antibiotics. Antimicrobial activity test results indicated that the inhibition zone diameters were ranged between 1.66 mm and 11.22 mm against various pathogens originated fish. The highest antimicrobial activity was recorded against the Aeromonas hydrophila ATCC 19570 (11.22 mm) for the MA-2 strain, while the lowest antimicrobial activity was determined against Vibrio anguillarum A4 (1.66 mm) for the MA-6 strain. L. gasseri MA-5, MA-3 and MA-6 showed alpha-amylase activity. The spectrophotometric and live-cell count data showed that all the strains exhibited resistance to sodium benzoate. The results suggest that L. gasseri MA strains could be good sources of probiotic for aquaculture and bio-protectives in seafood products. [ABSTRACT FROM AUTHOR]

• Different fishes were collected and were subjected to form an in vitro biofilm. • Huge array (up to 107 cfu/ml or g) of pathogenic bacteria. • Few of the isolates were sensitive and few were resistant against the antibiotics but after bio-film formation all the species acquired resistance. The ability of many bacteria to adhere on the host surfaces and forming biofilms has major implications in a wide variety of industries including the food industry, where biofilms may create a persistent source of contamination. In the same environmental condition, the multiple bacterial species can closely interact with each other and may easily enhance their drug resistance capability, which finally increases the multi-drug resistant (MDR) attribute of the species. The present study examined whether the mixed-species biofilm possesses any impact on the enhancement of the antibiotic resistance of the planktonic or single-cell bacterial isolates present in the fish samples. In this regard, Cyprinus rubrofuscus (Koi) , Heteropneustes fossilis (Shing) and Mystus vittatus (Tengra) fishes were collected and subjected to form an in vitro biofilm by shaking condition into the wise bath. The drug-resistant pattern was determined by the Kirby Bauer technique. All the samples exhibited a huge array (up to 107 cfu/ml or g) of bacteria such as E. coli , Klebsiella spp., Vibrio spp., Salmonella spp., Proteus spp. and Staphylococcus spp. The isolates from both the bulk samples and their corresponding biofilms were subjected to antibiogram assay using antibiotics such as Ampicillin (10 µg), Erythromycin (15 μg), Streptomycin (STP 10 μg), Oxacillin (10 µg), Nalidixic acid (30 µg). Before biofilm formation, few of the isolates were found to be sensitive and few were resistant against the antibiotics. But when the species were isolated from the biofilm the sensitive one acquired drug resistance and resistant strain unveiled more resistance towards the same antibiotics. The present study revealed extensive bacterial contamination in fish samples among those some were resistant against the supplied drugs. After the formation of multi-species biofilm, the isolates became more resistant against the same drugs that is alarming for consumers and major obstacles to maintain sustainable health. [ABSTRACT FROM AUTHOR]