Host Dependent-Transposon for a Plasmid Found in Aeromonas salmonicida subsp. salmonicida That Bears a catB3 Gene for Chloramphenicol Resistance

Plasmids that carry antibiotic resistance genes occur frequently in subsp. , an aquatic pathogen with severe consequences in salmonid farming. Here, we describe a 67 kb plasmid found in the subsp. 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 Tn family, but with an unusual organization. This transposon bears a gene (chloramphenicol resistance) that has not been detected yet in subsp. . The plasmid is transferable by conjugation into , but not into . Based on PCR analysis and genomic sequencing (Illumina and PacBio), we determined that the transposon is unstable in subsp. Strain SHY15-2939, but it is stable in 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 subsp. 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.