Search

Abstract Phage replication can be studied using various approaches, including measuring the optical density (OD) of a bacterial culture in a liquid medium in the presence of phages. A few quantitative methods are available to measure and compare the efficiency of phages by using a single index based on the analysis of OD curves. However, these methods are not always applicable to non-canonical OD curves. Using the concept of center of area (centroid), we developed a metric called Centroid Index (CI), sensitive to the trend of the growth curves (OD distribution) including bacterial regrowth, which is not considered by the methods already available. We also provide a user-friendly software to facilitate the calculation of CI. This method offers an alternative and more precise way to determine phage efficiency by considering the OD variations over time, which may help in the selection of phages for biocontrol applications.

Plasmids carrying antibiotic resistance genes occur frequently in Aeromonas salmonicida subsp. salmonicida, an aquatic pathogen. In this study, we describe three new plasmids found in strains of A. salmonicida subsp. salmonicida from Québec, Canada: pAsa-2358, pAsa-2900, and pAsa-2900b. The last two plasmids have a tet(D) resistance gene against tetracycline, which has not been previously found in A. salmonicida. The only difference between these two plasmids is the presence of a florfenicol resistance gene (floR) surrounded by two copies of the same insertion sequence, indicating that pAsa-2900 and pAsa-2900b are plasmid variants. All three plasmids represent an issue for aquaculture because they bear resistance genes against antibiotics used to treat infected fish: floR and tet(A) for pAsa-2358; floR, sul2, and tet(D) for pAsa-2900; and sul2 and tet(D) for pAsa-2900b. Conjugation assays confirmed that the three plasmids are capable of conjugation with one A. salmonicida mesophilic strain, one A. hydrophila strain, and Escherichia coli DH5α; except for pAsa-2358 that is not able to conjugate into E. coli DH5α. Based on the results of antibiotic resistance assays, the tet(D) gene provides a greater resistance to tetracycline than the tet(A) gene found in many A. salmonicida subsp. salmonicida plasmids. This is the case whether the plasmids are in their parental A. salmonicida subsp. salmonicida strains or in Aeromonas sp. transconjugates. The presence of these three plasmids in A. salmonicida subsp. salmonicida strengthens the role of this bacterium as a reservoir for antibiotic resistance genes.

ABSTRACT Chlamydia, comprising several human and zoonotic pathogens, is a genus of the conserved bacterial phylum Chlamydiota. Their obligate intracellular niche serves as a barrier for natural genetic exchange via horizontal gene transfer (HGT), and further limits the development and application of genetic tools. To date, the only example for recent inter-phylum HGT among the Chlamydiota is tetracycline resistance in the potentially zoonotic species Chlamydia suis, a close phylogenetic relative of human C. trachomatis, which causes bacterial sexually transmitted infections and ocular trachoma. Tetracycline resistance in porcine C. suis strains has been described worldwide and is always part of a genomic island dividing invasin (inv), located within a chromosomal region between the rRNA operon (rrn) and the nqrF reductase. Here, we aimed to expand the still modest number of available genetic manipulation systems for Chlamydia by generating allele-replacement and integration vectors for C. suis. These vectors comprised homologous C. suis sequences of the chromosomal region of interest, an E. coli origin of replication (ori) and selection markers but lacked the native chlamydial plasmids or its ori. We first recovered allele-replacement mutants using a vector that targets the tryptophan (trp) operon of C. suis. The vector was further successfully maintained as a free plasmid in C. trachomatis without allele replacement, suggesting complex plasmid dynamics in the absence of a chlamydial ori. Moreover, we showed that the hypervariable rrn-nqrF intergenic region of C. suis is highly susceptible to transformation, resulting in complete vector integration upstream of nqrF without interruption of the targeted inv gene. IMPORTANCE The obligate intracellular Chlamydia genus contains many pathogens with a negative impact on global health and economy. Despite recent progress, there is still a lack of genetic tools limiting our understanding of these complex bacteria. This study provides new insights into genetic manipulation of Chlamydia with the opportunistic porcine pathogen Chlamydia suis, the only chlamydial species naturally harboring an antibiotic resistance gene, originally obtained by horizontal gene transfer. C. suis is transmissible to humans, posing a potential public health concern. We report that C. suis can take up vectors that lack the native plasmid, a requirement for most chlamydial transformation systems described to date. Additionally, we show that C. trachomatis, the most common cause for bacterial sexually transmitted infections and infectious blindness worldwide, can be transformed with C. suis vectors. Finally, the chromosomal region that harbors the resistance gene of C. suis is highly susceptible to complete vector integration.

Abstract Background The quorum-sensing molecule farnesol, in opportunistic yeast Candida albicans, modulates its dimorphic switch between yeast and hyphal forms, and biofilm formation. Although there is an increasing interest in farnesol as a potential antifungal drug, the molecular mechanism by which C. albicans responds to this molecule is still not fully understood. Results A comparative genomic analysis between C. albicans strains that are naturally unresponsive to 30 µM of farnesol on TYE plates at 37 °C versus responsive strains uncovered new molecular determinants involved in the response to farnesol. While no signature gene was identified, amino acid changes in specific proteins were shown to correlate with the unresponsiveness to farnesol, particularly with substitutions in proteins known to be involved in the farnesol response. Although amino acid changes occur primarily in disordered regions of proteins, some amino acid changes were also found in known domains. Finally, the genomic investigation of intermediate-response strains showed that the non-response to farnesol occurs gradually following the successive accumulation of amino acid changes at specific positions. Conclusion It is known that large genomic changes, such as recombinations and gene flow (losses and gains), can cause major phenotypic changes in pathogens. However, it is still not well known or documented how more subtle changes, such as amino acid substitutions, play a role in the adaptation of pathogens. The present study shows that amino acid changes can modulate C. albicans yeast’s response to farnesol. This study also improves our understanding of the network of proteins involved in the response to farnesol, and of the involvement of amino acid substitutions in cellular behavior.

Aeromonas salmonicida subsp. salmonicida is a Gam-negative bacterium responsible for furunculosis in fish. Because this aquatic bacterial pathogen has a rich reservoir of antibiotic-resistant genes, it is essential to investigate antibacterial alternatives, including the use of phages. Yet, we have previously demonstrated the inefficiency of a phage cocktail designed against A. salmonicida subsp. salmonicida strains due to a phage resistance phenotype associated to a prophage, namely Prophage 3. To bypass this resistance, one of the solutions is to isolate novel phages capable of infecting Prophage 3-bearing strains. Here we report on the isolation and characterization of the new virulent phage vB_AsaP_MQM1 (or MQM1), which is highly specific to A. salmonicida subsp. salmonicida strains. Phage MQM1 inhibited the growth of 01-B516, a strain carrying Prophage 3, including when combined to the previous phage cocktail. MQM1 infected 26 out of the 30 (87%) Prophage 3-bearing strains tested. Its linear dsDNA genome contains 63,343 bp, with a GC content of 50.2%. MQM1 genome can encode 88 proteins and 8 tRNAs, while no integrase or transposase-encoding genes were found. This podophage has an icosahedral capsid and a non-contractile short tail. We suggest that MQM1 may be a good addition to future phage cocktails against furunculosis to resolve the Prophage 3-resistance issue.

The plasmid known as pAsa5 is present in Aeromonas salmonicida subsp. salmonicida, a fish pathogen. The pAsa5 plasmid carries genes that are essential for the bacterium’s virulence. Recombination events are known to occur in pAsa5, resulting in the loss of certain segments or the acquisition of additional genetic elements. For example, the transposon carried by the large pAsa8 plasmid was found to be inserted into the pAsa5 plasmid in the SHY16-3432 strain, enabling the addition of antibiotic resistance genes to this plasmid, which does not normally possess any. In this study, we present the isolation of additional strains carrying pAsa8. Further analyses of these strains revealed that a fusion between pAsa5 and the complete version of pAsa8 is possible. The pAsa8 transposon insertion in pAsa5 seen in the SHY16-3432 strain appears to be an aberrant event compared to the fusion of the two full-length plasmids. A 22-nucleotide sequence, present in both plasmids, serves as the site for the fusion of the two plasmids. Moreover, it is possible to introduce pAsa8 through conjugation into naive strains of A. salmonicida subsp. salmonicida and once the plasmid is within a new strain, the fusion with pAsa5 is detectable. This study reveals a previously unexplored aspect of pAsa5 plasmid biology, highlighting an additional risk for the spread of antibiotic resistance genes in A. salmonicida subsp. salmonicida.

Abstract Objective This study was designed to find a method to enhance the recovery of Cryptosporidium spp. and Giardia spp. parasites from water samples for research purposes compared to the results that can be achieved with USEPA Method 1623.1. Four different approaches were used to test water samples that were artificially spiked with parasites. The approaches were: (i) Method 1623.1 itself, (ii) elution of Method 1623.1 combined with microfiltration, (iii) an elution technique based on grinding the filter membrane in a blender before the eluent was concentrated by immunomagnetic separation, and (iv) the blender elution followed by microfiltration. Fluorescence microscopy was used to determine which approach led to the highest parasite recovery rates. Results Method 1623.1 gave the best results for Giardia, while all four approaches were statistically equivalent for Cryptosporidium. We evaluated the costs and laboratory time requirements for each protocol to give readers a complete comparison of the methods tested. Elution of Method 1623.1 combined with microfiltration resulted in lower costs and less laboratory work time without compromising the recovery of the parasites.

For a long time, the bacterial species Aeromonas salmonicida seemed to be limited to a regrouping of psychrophilic subspecies that infect fish, particularly salmonids [...]

Plasmids that carry antibiotic resistance genes occur frequently in Aeromonas salmonicida subsp. salmonicida, an aquatic pathogen with severe consequences in salmonid farming. Here, we describe a 67 kb plasmid found in the A. salmonicida subsp. salmonicida Strain SHY15-2939 from Quebec, Canada. This new plasmid, named pAsa-2939 and identified by high throughput sequencing, displays features never found before in this bacterial species. It contains a transposon related to the Tn21 family, but with an unusual organization. This transposon bears a catB3 gene (chloramphenicol resistance) that has not been detected yet in A. salmonicida subsp. salmonicida. The plasmid is transferable by conjugation into Aeromonas hydrophila, but not into Escherichia coli. Based on PCR analysis and genomic sequencing (Illumina and PacBio), we determined that the transposon is unstable in A. salmonicida subsp. salmonicida Strain SHY15-2939, but it is stable in A. hydrophila trans-conjugants, which explains the chloramphenicol resistance variability observed in SHY15-2939. These results suggest that this bacterium is likely not the most appropriate host for this plasmid. The presence of pAsa-2939 in A. salmonicida subsp. salmonicida also strengthens the reservoir role of this bacterium for antibiotic resistance genes, even those that resist antibiotics not used in aquaculture in Québec, such as chloramphenicol.

Aeromonas salmonicida subsp. salmonicida is a Gram-negative bacterium that causes furunculosis, a fish disease claiming substantial economic losses in the aquaculture industry. Major challenges exist in monitoring and controlling fish infections in aquaculture farms. Development of management practices to improve the sustainability of fish farming with disease prevention necessitates studies using well-defined systems and well-characterized bacterial isolates. Even though several A. salmonicida subsp. salmonicida genomes have been completely assembled and thoroughly annotated, in vivo pathogenicity data are lacking. Here we present A. salmonicida subsp. salmonicida 890054 as a prototype strain for standardized furunculosis challenges with survival data. Computational analysis of sequencing results provided a complete circular genome with annotations of plasmids carrying virulence factors, antimicrobial resistance, and secondary metabolite coding genes. The analysis also revealed the presence of an IncU plasmid distinct from other IncU plasmids previously associated with Aeromonas.

Aeromonas salmonicida subsp. salmonicida is a pathogenic bacterium responsible for furunculosis in salmonids. Following an outbreak of furunculosis, the infection can be treated with antibiotics, but it is common to observe ineffective treatment due to antibiotic resistance. This bacterium has a wide variety of plasmids responsible for this resistance. Among them, pRAS3 carries a tetracycline resistance gene. Several variants of this plasmid have been discovered over the years (pRAS3-3432 and pRAS3.1 to 3.4). During the present study, two new variants of the plasmid pRAS3 were identified (pRAS3.5 and pRAS3-3759) in strains of A. salmonicida subsp. salmonicida. Plasmid pRAS3-3759, which has been found in many strains from the same region over the past three years, has an additional genetic element identical to one found in pRAS3-3432. This genetic element was also found in Chlamydia suis, a swine pathogen. In this study, we analyzed the bacteria’s resistance to tetracycline, the number of copies of the plasmids, and the growth of the strains that carry five of the pRAS3 variants (pRAS3.3 to 3.5, pRAS3-3432, and pRAS3-3759). The results show no particular trend despite the differences between the plasmids, except for the resistance to tetracycline when analyzed in an isogenic background. Blast analysis also revealed the presence of pRAS3 plasmids in other bacterial species, which suggests that this plasmid family has widely spread. This study once again highlights the ability of A. salmonicida subsp. salmonicida to adapt to furunculosis antibiotic treatments, and the still-growing family of pRAS3 plasmids.

Antimicrobial resistance (AMR) is continuing to grow across the world. Though often thought of as a mostly public health issue, AMR is also a major agricultural and environmental problem. As such, many researchers refer to it as the preeminent One Health issue. Aerial transport of antimicrobial-resistant bacteria via bioaerosols is still poorly understood. Recent work has highlighted the presence of antibiotic resistance genes in bioaerosols. Emissions of AMR bacteria and genes have been detected from various sources, including wastewater treatment plants, hospitals, and agricultural practices; however, their impacts on the broader environment are poorly understood. Contextualizing the roles of bioaerosols in the dissemination of AMR necessitates a multidisciplinary approach. Environmental factors, industrial and medical practices, as well as ecological principles influence the aerial dissemination of resistant bacteria. This article introduces an ongoing project assessing the presence and fate of AMR in bioaerosols across Canada. Its various sub-studies include the assessment of the emissions of antibiotic resistance genes from many agricultural practices, their long-distance transport, new integrative methods of assessment, and the creation of dissemination models over short and long distances. Results from sub-studies are beginning to be published. Consequently, this paper explains the background behind the development of the various sub-studies and highlight their shared aspects.

Among the major issues linked with producing safe water for consumption is the presence of the parasitic protozoa Cryptosporidium spp. and Giardia spp. Since they are both responsible for gastrointestinal illnesses that can be waterborne, their monitoring is crucial, especially in water sources feeding treatment plants. Although their discovery was made in the early 1900s and even before, it was only in 1999 that the U.S. Environmental Protection Agency (EPA) published a standardized protocol for the detection of these parasites, modified and named today the U.S. EPA 1623.1 Method. It involves the flow-through filtration of a large volume of the water of interest, the elution of the biological material retained on the filter, the purification of the (oo)cysts, and the detection by immunofluorescence of the target parasites. Since the 1990s, several molecular-biology-based techniques were also developed to detect Cryptosporidium and Giardia cells from environmental or clinical samples. The application of U.S. EPA 1623.1 as well as numerous biomolecular methods are reviewed in this article, and their advantages and disadvantages are discussed guiding the readers, such as graduate students, researchers, drinking water managers, epidemiologists, and public health specialists, through the ever-expanding number of techniques available in the literature for the detection of Cryptosporidium spp. and Giardia spp. in water.

Microbiology literacy is essential for any world citizen, considering the major impacts that microorganisms have on our daily lives and health. Consequently, we have created a complete microbiology course available to the non-microbiology students at our university. This course, entitled "Microbes and You" is fully accessible online (in French only) and helps students advance their knowledge of microorganisms in a fun way. Before the course, most students are only aware of the negative role of microorganisms on our health, and this course teaches them that our relationship with microbes is more complex and possesses many advantages. In this article, we share our experience and the extremely positive response of the students. We hope to encourage other microbiologists to set up basic courses in microbiology for non-scientists, or scientists without basic knowledge in this science.

Technological innovations and quality control processes within blood supply organizations have significantly improved blood safety for both donors and recipients. Nevertheless, the risk of transfusion-transmitted infection remains non-negligible. Applying a nanoparticular, antibacterial coating at the surface of medical devices is a promising strategy to prevent the spread of infections. In this study, we characterized the antibacterial activity of an SiO2 nanoparticular coating (i.e., the “Medical Antibacterial and Antiadhesive Coating” [MAAC]) applied on relevant polymeric materials (PM) used in the biomedical field. Electron microscopy revealed a smoother surface for the MAAC-treated PM compared to the reference, suggesting antiadhesive properties. The antibacterial activity was tested against selected Gram-positive and Gram-negative bacteria in accordance with ISO 22196. Bacterial growth was significantly reduced for the MAAC-treated PVC, plasticized PVC, polyurethane and silicone (90–99.999%) in which antibacterial activity of ≥1 log reduction was reached for all bacterial strains tested. Cytotoxicity was evaluated following ISO 10993-5 guidelines and L929 cell viability was calculated at ≥90% in the presence of MAAC. This study demonstrates that the MAAC could prevent bacterial contamination as demonstrated by the ISO 22196 tests, while further work needs to be done to improve the coating processability and effectiveness of more complex matrices.

Aeromonas salmonicida is a global distributed Gram-negative teleost pathogen, affecting mainly salmonids in fresh and marine environments. A. salmonicida strains are classified as typical or atypical depending on their origin of isolation and phenotype. Five subspecies have been described, where A. salmonicida subsp. salmonicida is the only typical subspecies, and the subsp. achromogenes, masoucida, smithia, and pectinolytica are considered atypical. Genomic differences between A. salmonicida subsp. salmonicida isolates and their relationship with the current classification have not been explored. Here, we sequenced and compared the complete closed genomes of four virulent strains to elucidate their molecular diversity and pathogenic evolution using the more accurate genomic information so far. Phenotypes, biochemical, and enzymatic profiles were determined. PacBio and MiSeq sequencing platforms were utilized for genome sequencing. Comparative genomics showed that atypical strains belong to the subsp. salmonicida, with 99.55% ± 0.25% identity with each other, and are closely related to typical strains. The typical strain A. salmonicida J223 is closely related to typical strains, with 99.17% identity with the A. salmonicida A449. Genomic differences between atypical and typical strains are strictly related to insertion sequences (ISs) activity. The absence and presence of genes encoding for virulence factors, transcriptional regulators, and non-coding RNAs are the most significant differences between typical and atypical strains that affect their phenotypes. Plasmidome plays an important role in A. salmonicida virulence and genome plasticity. Here, we determined that typical strains harbor a larger number of plasmids and virulence-related genes that contribute to its acute virulence. In contrast, atypical strains harbor a single, large plasmid and a smaller number of virulence genes, reflected by their less acute virulence and chronic infection. The relationship between phenotype and A. salmonicida subspecies’ taxonomy is not evident. Comparative genomic analysis based on completed genomes revealed that the subspecies classification is more of a reflection of the ecological niche occupied by bacteria than their divergences at the genomic level except for their accessory genome.

The bacterium Aeromonas salmonicida has long been known to be one of the most feared pathogens in fish farming. However, the more we discover about this bacterial species, the more we question whether it is really exclusively an aquatic pathogen. In recent years, it has become obvious that this bacterial species includes a myriad of strains with various lifestyle and ecological niches, including the well-known strict psychrophiles, the first bacteria known of the species, and the newly described mesophilic strains. The mesophiles are able to grow at low temperatures, but even better at temperatures of approximately 37 °C, which strict psychrophiles cannot do. In this perspective article, we address some aspects surrounding this dual lifestyle in A. salmonicida, including the impact of mobile genetic elements, and how future research around this bacterial species may focus on the psychrophilic/mesophilic dichotomy, which makes A. salmonicida an increasingly interesting and relevant model for the study of speciation.

Abstract Objective Pseudomonas aeruginosa is an opportunistic bacterial pathogen well known to cause chronic lung infections in individuals with cystic fibrosis (CF). Some strains adapted to this particular niche show distinct phenotypes, such as biofilm hyperproduction. It is necessary to study CF clinical P. aeruginosa isolates, such as Liverpool Epidemic Strains (LES), to acquire a better understanding of the key genes essential for in vivo maintenance and the major virulence mechanisms involved in CF lung infections. Previously, a library of 9216 mutants of the LESB58 strain were generated by signature-tagged mutagenesis (STM) and screened in the rat model of chronic lung infection, allowing the identification of 163 STM mutants showing defects in in vivo maintenance. Results In the present study, these 163 mutants were successively screened in two additional surrogate host models (the amoeba and the fruit fly). The STM PALES_11731 mutant was the unique non-virulent in the three hosts. A competitive index study in rat lungs confirmed that the mutant was 20-fold less virulent than the wild-type strain. This study demonstrated the pertinence to use a multi-host approach to study the genetic determinants of P. aeruginosa strains infecting CF patients.

Aquaculture is a rapidly growing food production sector. Fish farmers are experiencing increasing problems with antibiotic resistance when fighting against pathogenic bacteria such as Aeromonas salmonicida subsp. salmonicida, the causative agent of furunculosis. Phage therapy may provide an alternative, but effective use must be determined. Here, we studied the inhibition of A. salmonicida subsp. salmonicida strains by five phages (HER98 [44RR2.8t.2], HER110 [65.2], SW69-9, L9-6 and Riv-10) used individually or as combinations of two to five phages. A particular combination of four phages (HER98 [44RR2.8t.2], SW69-9, Riv-10, and HER110 [65.2]) was found to be the most effective when used at an initial multiplicity of infection (MOI) of 1 against the A. salmonicida subsp. salmonicida strain 01-B526. The same phage cocktail is effective against other strains except those bearing a prophage (named Prophage 3), which is present in 2/3 of the strains from the province of Quebec. To confirm the impact of this prophage, we tested the effectiveness of the same cocktail on strains that were either cured or lysogenized with Prophage 3. While the parental strains were sensitive to the phage cocktail, the lysogenized ones were much less sensitive. These data indicate that the prophage content of A. salmonicida subsp. salmonicida can affect the efficacy of a cocktail of virulent phages for phage therapy purposes.

Abstract Phages infecting Aeromonas salmonicida subsp. salmonicida, the causative agent of the fish disease furunculosis, have been isolated for decades but very few of them have been characterized. Here, the host range of 12 virulent phages, including three isolated in the present study, was evaluated against a panel of 65 A. salmonicida isolates, including representatives of the psychrophilic subspecies salmonicida, smithia, masoucida, and the mesophilic subspecies pectinolytica. This bacterial set also included three isolates from India suspected of being members of a new subspecies. Our results allowed to elucidate a lytic dichotomy based on the lifestyle of A. salmonicida (mesophilic or psychrophilic) and more generally, on phage types (lysotypes) for the subspecies salmonicida. The genomic analyses of the 12 phages from this study with those available in GenBank led us to propose an A. salmonicida phage pan-virome. Our comparative genomic analyses also suggest that some phage genes were under positive selection and A. salmonicida phage genomes having a discrepancy in GC% compared to the host genome encode tRNA genes to likely overpass the bias in codon usage. Finally, we propose a new classification scheme for A. salmonicida phages.

Abstract Background Aeromonas salmonicida subsp. salmonicida is a ubiquitous psychrophilic waterborne bacterium and a fish pathogen. The numerous mobile elements, especially insertion sequences (IS), in its genome promote rearrangements that impact its phenotype. One of the main virulence factors of this bacterium, its type three secretion system (TTSS), is affected by these rearrangements. In Aeromonas salmonicida subsp. salmonicida most of the TTSS genes are encoded in a single locus on a large plasmid called pAsa5, and may be lost when the bacterium is cultivated at a higher temperature (25 °C), producing non-virulent mutants. In a previous study, pAsa5-rearranged strains that lacked the TTSS locus on pAsa5 were produced using parental strains, including 01-B526. Some of the generated deletions were explained by homologous recombination between ISs found on pAsa5, whereas the others remained unresolved. To investigate those rearrangements, short- and long-read high-throughput sequencing technologies were used on the A. salmonicida subsp. salmonicida 01-B526 whole genome. Results Whole genome sequencing of the 01-B526 strain revealed that its pAsa5 has an additional IS copy, an ISAS5, compared to the reference strain (A449) sequence, which allowed for a previously unknown rearrangement to occur. It also appeared that 01-B526 bears a second large plasmid, named pAsa9, which shares 40 kbp of highly similar sequences with pAsa5. Following these discoveries, previously unexplained deletions were elucidated by genotyping. Furthermore, in one of the derived strains a fusion of pAsa5 and pAsa9, involving the newly discovered ISAS5 copy, was observed. Conclusion The loss of TTSS and hence virulence is explained by one consistent mechanism: IS-driven homologous recombination. The similarities between pAsa9 and pAsa5 also provide another example of genetic diversity driven by ISs.

Protists are key stone components of aquatic ecosystems, sustaining primary productivity and aquatic food webs. However, their diversity, ecology and structuring factors shaping their temporal distribution remain strongly misunderstood in freshwaters. Using high-throughput sequencing on water samples collected over 16 different months (including two summer and two winter periods), combined with geochemical measurements and climate monitoring, we comprehensively determined the pico- and nanoeukaryotic community composition and dynamics in a Canadian river undergoing prolonged ice-cover winters. Our analysis revealed a large protist diversity in this fluctuating ecosystem and clear seasonal patterns demonstrating a direct and/or indirect selective role of abiotic factors, such as water temperature or nitrogen concentrations, in structuring the eukaryotic microbial community. Nonetheless, our results also revealed that primary productivity, predatory as well as parasitism lifestyles, inferred from fine phylogenetic placements, remained potentially present over the annual cycle, despite the large seasonal fluctuations and the remodeling of the community composition under ice. In addition, potential interplays with the bacterial community composition were identified supporting a possible contribution of the bacterial community to the temporal dynamics of the protist community structure. Our results illustrate the complexity of the eukaryotic microbial community and provide a substantive and useful dataset to better understand the global freshwater ecosystem functioning.

The type three secretion system (TTSS) locus of Aeromonas salmonicida subsp. salmonicida, located on the plasmid pAsa5, is known to be lost when the bacterium is grown at temperatures of 25 °C. The loss of the locus is due to the recombination of the insertion sequences flanking the TTSS region. However, the mechanism involved in this recombination is still elusive. Here, we analyzed 22 A. salmonicida subsp. salmonicida strains that had already lost their TTSS locus, and we systematically explored another 47 strains for their susceptibility to lose the same locus when grown at 25 °C. It appeared that strains from Europe were more prone to lose their TTSS locus compared to Canadian strains. More specifically, it was not possible to induce TTSS loss in Canadian strains that have AsaGEI2a, a genomic island, and prophage 3, or in Canadian strains without a genomic island. A comparative genomic approach revealed an almost perfect correlation between the presence of a cluster of genes, not yet characterized, and the susceptibility of various groups of strains to lose their locus. This cluster of genes encodes putative proteins with DNA binding capacity and phage proteins. This discovery creates new opportunities in the study of pAsa5 thermosensitivity.

Streams and rivers convey freshwater from lands to the oceans, transporting various organic particles, minerals, and living organisms. Microbial communities are key components of freshwater food webs and take up, utilize, and transform this material. However, there are still important gaps in our understanding of the dynamic of these organisms along the river channels. Using high-throughput 16S and 18S rRNA gene sequencing and quantitative PCR on a 11-km long transect of the Saint-Charles River (Quebec, CA), starting from its main source, the Saint-Charles Lake, we show that bacterial and protist community structures in the river drifted quickly but progressively downstream of its source. The dominant Operational Taxonomic Units (OTUs) of the lake, notably related to Cyanobacteria, decreased in proportions, whereas relative proportions of other OTUs, such as a Pseudarcicella OTU, increased along the river course, becoming quickly predominant in the river system. Both prokaryotic and protist communities changed along the river transect, suggesting a strong impact of the shift from a stratified lake ecosystem to a continuously mixed river environment. This might reflect the cumulative effects of the increasing water turbulence, fluctuations of physicochemical conditions, differential predation pressure in the river, especially in the lake outlet by benthic filter feeders, or the relocation of microorganisms, through flocculation, sedimentation, resuspension, or inoculation from the watershed. Our study reveals that the transit of water in a river system can greatly impact both bacterial and micro-eukaryotic community composition, even over a short distance, and, potentially, the transformation of materials in the water column.

Protozoa are natural predators of bacteria, but some bacteria can evade digestion once phagocytosed. Some of these resistant bacteria can be packaged in the fecal pellets produced by protozoa, protecting them from physical stresses and biocides. Depending on the bacteria and protozoa involved in the packaging process, pellets can have different morphologies. In the present descriptive study, we evaluated the packaging process with 20 bacteria that have never been tested before for packaging by ciliates. These bacteria have various characteristics (shape, size, Gram staining). All of them appear to be included in pellets produced by the ciliates Tetrahymena pyriformis and/or T. thermophila in at least one condition tested. We then focused on the packaging morphology of four of these bacteria. Our results demonstrated that, as shown previously for Mycobacterium smegmatis, the packaging of Microbacterium oxydans, Micrococcus luteus, and Cupriavidus sp. was formed of a single layer of material. The packaging of Cellulosimicrobiumfunkei was made of indistinguishable material. A different pellet morphology was obtained for each of the four bacterial strains studied. The ingestion of small bacteria resulted in rounder, denser, and more regular pellets. These results support the idea that bacteria packaging is a relatively widespread phenomenon.

Aeromonas salmonicida is a Gram-negative bacterium, known as a fish pathogen since its discovery. Although the species was initially considered psychrophilic, a mesophilic subspecies (pectinolytica) and many other mesophilic strains still not attributed to subspecies have been described in the last two decades. These mesophilic strains were sampled from various sources, including humans, and some of them are known to be pathogenic. In this study, we describe a strain, JF2480, which was isolated from the spleen, and also found the kidney and liver of a dead pied avocet (Recurvirostra avosetta), a type of migratory bird inhabiting aquatic environments. A core genome phylogenomic analysis suggests that JF2480 is taxonomically distant from other known A. salmonicida subspecies. The genome sequence confirms that the strain possesses key virulence genes that are present in the typical A. salmonicida psychrophilic subspecies, with the exception of the genes encoding the type three secretion system (T3SS). Bacterial virulence assays conducted on the surrogate host Dictyostelium discoideum amoeba confirmed that the strain is virulent despite the lack of T3SS. Bacterial growth curves showed that strain JF2480 grow well at 40 °C, the body temperature of the pied avocet, and even faster at 41 °C, compared to other mesophilic strains. Discovery of this strain further demonstrates the extent of the phylogenomic tree of this species. This study also suggests that A. salmonicida can infect a wider array of hosts than previously suspected and that we need to rethink the way we perceive A. salmonicida’s natural environment.

In Aeromonas salmonicida subsp. salmonicida, a bacterium that causes fish disease, there are two types of small plasmids (

Aeromonas sobria is a mesophilic motile aeromonad currently depicted as an opportunistic pathogen, despite increasing evidence of mutualistic interactions in salmonid fish. However, the determinants of its host-microbe associations, either mutualistic or pathogenic, remain less understood than for other aeromonad species. On one side, there is an over-representation of pathogenic interactions in the A. sobria literature, of which only three articles to date report mutualistic interactions; on the other side, genomic characterization of this species is still fairly incomplete as only two draft genomes were published prior to the present work. Consequently, no study specifically investigated the biodiversity of A. sobria. In fact, the investigation of A. sobria as a species complex may have been clouded by: (i) confusion with A. veronii biovar sobria because of their similar biochemical profiles, and (ii) the intrinsic low resolution of previous studies based on 16S rRNA gene sequences and multilocus sequence typing. So far, the only high-resolution, phylogenomic studies of the genus Aeromonas included one A. sobria strain (CECT 4245 / Popoff 208), making it impossible to robustly conclude on the phylogenetic intra-species diversity and the positioning among other Aeromonas species. To further understand the biodiversity and the spectrum of host-microbe interactions in A. sobria as well as its potential genomic diversity, we assessed the genomic and phenotypic heterogeneity among five A. sobria strains: two clinical isolates recovered from infected fish (JF2635 and CECT 4245), one from an infected amphibian (08005) and two recently isolated brook charr probionts (TM12 and TM18) which inhibit in vitro growth of A. salmonicida subsp. salmonicida (a salmonid fish pathogen). A phylogenomic assessment including 2,154 softcore genes corresponding to 946,687 variable sites from 33 Aeromonas genomes confirms the status of A. sobria as a distinct species divided in two subclades, with 100% bootstrap support. The phylogenomic split of A. sobria in two subclades is corroborated by a deep dichotomy between all five A. sobria strains in terms of inhibitory effect against A. salmonicida subsp. salmonicida, gene contents and codon usage. Finally, the antagonistic effect of A. sobria strains TM12 and TM18 suggests novel control methods against A. salmonicida subsp. salmonicida.

Bovine gammaherpesvirus 4 (BoHV-4) is a herpesvirus widespread in cattle populations, and with no clear disease association. Its genome contains a long unique coding region (LUR) flanked by polyrepetitive DNA and 79 open reading frames (ORFs), with unique 17 ORFs, named Bo1 to Bo17. In 2009, a BoHV-4 strain was isolated (FMV09-1180503: BoHV-4-FMV) from cattle with respiratory disease from Quebec, Canada, and its LUR was sequenced. Despite the overall high similarity, BoHV-4-FMV had the most divergent LUR sequence compared to the two known BoHV-4 reference strain genomes; most of the divergences were in the Bo genes and in the repeat regions. Our phylogenetic analysis based on DNA polymerase and thymidine kinase genes revealed that virus isolate was BoHV-4 gammaherpesvirus and clustered it together with European BoHV-4 strains. Because BoHV-4-FMV was isolated from animals presenting respiratory signs, we have updated the BoHV-4 Canadian cattle seroprevalence data and tried to find out whether there is a link between clinical manifestation and BoHV-4 seropositivity. An indirect immunofluorescence assay (IFA) was performed with nearly 200 randomized sera of dairy cattle from two Canadian provinces, Quebec (n = 100) and Ontario (n = 91). An additional set of sera obtained from Quebec, from the healthy (n = 48) cows or from the animals experiencing respiratory or reproductive problems (n = 75), was also analyzed by IFA. BoHV-4 seroprevalence in Canadian dairy cattle was 7.9% (Quebec: 6% and Ontario: 9.9%). Among animals from the Quebec-based farms, diseased animals showed higher BoHV-4 seropositivity than healthy animals (P

Aeromonas salmonicida subsp. salmonicida is a Gram-negative bacterium causing furunculosis, an opportunistic infection of farmed salmonid fish. Current treatment methods against furunculosis rely heavily on antibiotherapy. However, strains of this opportunistic fish pathogen were found to possess genes that confer resistance to major antibiotics including those used to cure furunculosis. Therefore, dispensing bacterial symbionts as probiotics to susceptible hosts appears to be a promising alternative. Here, we present the genomic characterization and in vivo safety assessment of two brook charr (Salvelinus fontinalis) bacterial symbionts that inhibited A. salmonicida subsp. salmonicida growth in vitro (Pseudomonas fluorescens ML11A and Aeromonas sobria TM18) as well as a commercialized probiotic, Pediococcus acidilactici MA18/5M (Bactocell®). The genomic sequences of ML11A and TM18 obtained by whole-genome shotgun sequencing lack key virulence factor genes found in related pathogenic strains. Their genomic sequences are also devoid of genes involved in the inactivation (or target modification of) several key antimicrobial compounds used in salmonid aquaculture. Finally, when administered daily to live brook charr fingerlings, ML11A, TM18 and Bactocell® helped improve several physiological condition metrics such as mean body weight, Fulton’s condition factor and blood plasma lysozyme activity (an indicator for innate immune activity).

Aeromonas salmonicida subsp. salmonicida, the causative agent of furunculosis in salmonids, is an issue especially because many isolates of this bacterium display antibiotic resistances, which limit treatments against the disease. Recent results suggested the possible existence of alternative forms of pAsa4, a large plasmid found in A. salmonicida subsp. salmonicida and bearing multiple antibiotic resistance genes. The present study reveals the existence of two newly detected pAsa4 variants, pAsa4b and pAsa4c. We present the extensive characterization of the genomic architecture, the mobile genetic elements and the antimicrobial resistance genes of these plasmids in addition to the reference pAsa4 from the strain A449. The analysis showed differences between the three architectures with consequences on the content of resistance genes. The genomic plasticity of the three pAsa4 variants could be partially explained by the action of mobile genetic elements like insertion sequences. Eight additional isolates from Canada and Europe that bore similar antibiotic resistance patterns as pAsa4-bearing strains were genotyped and specific pAsa4 variants could be attributed to phenotypic profiles. pAsa4 and pAsa4c were found in Europe, while pAsa4b was found in Canada. In accordance with their content in conjugative transfer genes, only pAsa4b and pAsa4c can be transferred by conjugation in Escherichia coli. The plasticity of pAsa4 variants related to the acquisition of antibiotic resistance indicates that these plasmids may pose a threat in terms of the dissemination of antimicrobial-resistant A. salmonicida subsp. salmonicida bacteria.