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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]

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]

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]

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]

We report the draft genome sequence from Aeromonas salmonicida sp. strain CBA100, which was characterized as an antibiotic-resistant bacterium isolated from infected rainbow trout. The total size of the genome is 4 788 109 bp, with a G + C content of 60.55%. Comparison of its open reading frames shows that the closest homologue to one third of the genes of strain CBA100 are found in A. hydrophila. The strain contains several efflux pumps and putative genes that confer resistance to multiclass antibiotics, including macrolide, β-lactamics, florfenicol and quinolones. The antibiogram profile suggests that efflux pumps are the main mechanism of resistance to non-β-lactamic antibiotics. This is the first genome of a Chilean isolate of A. salmonicida, which should shed light on the design of strain-specific vaccines against this pathogen and reduce the use of antibiotics for preventive treatment in Chilean aquaculture. [ABSTRACT FROM AUTHOR]

The main objective of this study was to characterize the tet(X) genes, which encode a monooxygenase that catalyzes the degradation of tetracycline antibiotics, carried by the resistant strains FP105 and FP233-J200, using whole-genome sequencing analysis. The isolates were recovered from fin lesion and kidney samples of diseased rainbow trout Oncorhynchus mykiss, during two Flavobacteriosis outbreaks occurring in freshwater farms located in Southern Chile. The strains were identified as Epilithonimonas spp. by using biochemical tests and by genome comparison analysis using the PATRIC bioinformatics platform and exhibited a minimum inhibitory concentration (MIC) of oxytetracycline of 128 µg/mL. The tet(X) genes were located on small contigs of the FP105 and FP233-J200 genomes. The sequences obtained for the tet(X) genes and their genetic environment were compared with the genomes available in the GenBank database of strains of the Chryseobacterium clade belonging to the Flavobacterium family, isolated from fish and carrying the tet(X) gene. The Tet(X) proteins synthesized by the Chilean Epilithonimonas strains showed a high amino acid similarity (range from 84% to 100%), with the available sequences found in strains belonging to the genus Chryseobacterium and Flavobacterium isolated from fish. An identical neighborhood of tet(X) genes from both Chilean strains was observed. The genetic environment of tet(X) observed in the two strains of Epilithonimonas studied was characterized by the upstream location of a sequence encoding a hypothetical protein and a downstream located alpha/beta hydrolase-encoding gene, similar to the observed in some of the tet(X) genes carried by Chryseobacterium and Flavobacterium strains isolated from fish, but the produced proteins exhibited a low amino acid identity (25–27%) when compared to these synthesized by the Chilean strains. This study reports for the first time the carriage of the tet(X) gene by the Epilithonimonas genus and their detection in fish pathogenic bacteria isolated from farmed salmonids in Chile, thus limiting the use of therapies based on oxytetracycline, the antimicrobial most widely used in Chilean freshwater salmonid farming. This results suggest that pathogenic strains of the Chryseobacterium clade occurring in Chilean salmonid farms may serve as important reservoirs of tet(X) genes. [ABSTRACT FROM AUTHOR]

Cabello and Godfrey (2019) recently published the review "Salmon aquaculture, Piscirickettsia salmonis virulence, and One Health: Dealing with harmful synergies between heavy antimicrobial use and piscine and human health," in Aquaculture. Several gaps in the review hinder readers from understanding the complexity of P. salmonis and the bacterium-host interaction. This is in addition to omitting essential information that, in my opinion, should be reported because erroneous policy, economic, and social decisions could be made and could notably impact the Chilean aquaculture industry. For example, P. salmonis is not an opportunistic pathogen with poor virulence/pathogenicity, and the literature instead widely supports several pathogenic mechanisms (e.g., iron uptake) and virulence capacity in vitro and in vivo. Furthermore, P. salmonis is not an emerging pathogen – P. salmonis was described 31 years ago, is currently the primary cause for salmonid mortalities in Chile, and is even officially monitored as a national health threat. Yet other examples reveal gross bias, such as indicating that quinolones are a current treatment for P. salmonis. In reality, the use of quinolones have drastically decreased since 2015 according to official government reports (<1% total volume depending on the year), and prior use is not only against piscirickettsiosis but also the bacterium responsible for bacterial kidney disease. The scope of this commentary addresses all of these issues in Cabello and Godfrey's (2019) review. [ABSTRACT FROM AUTHOR]