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The ubiquitous water-borne Gram-negative bacterium Aeromonas salmonicida subsp. salmonicida is the causative agent of furunculosis, a worldwide disease in fish farms. Plasmids carrying antibiotic resistance genes have already been described for this bacterium. The aim of the present study was to identify and characterize additional multidrug resistance plasmids in A. salmonicida subsp. salmonicida. We sequenced the plasmids present in two multiple antibiotic-resistant isolates using highthroughput technologies. We also investigated 19 other isolates with various multidrug resistance profiles by genotyping PCR and assessed their resistance to tetracycline. We identified variants of the pAB5S9 and pSN254 plasmids that carry several antibiotic resistance genes and that have been previously reported in bacteria other than A. salmonicida subsp. salmonicida, which suggests a high level of interspecies exchange. Genotyping analyses and the antibiotic resistance profiles of the 19 other isolates support the idea that multiple versions of pAB5S9 and pSN254 exist in A. salmonicida subsp. salmonicida. We also identified variants of the pRAS3 plasmid. The present study revealed that A. salmonicida subsp. salmonicida harbors a wide variety of plasmids, which suggests that this ubiquitous bacterium may contribute to the spread of antibiotic resistance genes in the environment. The Gram-negative bacterium Aeromonas salmonicida subsp. salmonicida is an opportunistic fish pathogen (1). It is the etiological agent of furunculosis, a disease that especially affects salmonids in fish farms (2). While antibiotics are commonly used to treat A. salmonicida subsp. salmonicida infections, multidrug-resistant isolates have been frequently detected (3-5), preventing the effective treatment of furunculosis.Many fully characterized plasmids from A. salmonicida subsp. salmonicida have provided antibiotic resistance to this species (2). All the known plasmids in A. salmonicida subsp. salmonicida harboring antibiotic resistance genes include at least a tetracycline resistance gene. The vast majority of the plasmids bearing antibiotic resistance genes confer multiple types of resistance to A. salmonicida subsp. salmonicida, including the large (167-kb) plasmid pAsa4, which provides resistance against chloramphenicol, spectinomycin, streptomycin, sulfonamides, tetracycline, mercury, and quaternary ammonium compounds (6). A plasmid bearing multiple resistance genes that is similar to the large pSN254 plasmid in Salmonella enterica (7) has been partially described in A. salmonicida subsp. salmonicida (3). This pSN254-like plasmid can be transferred via conjugation from A. salmonicida subsp. salmonicida to multiple receivers, including Escherichia coli, Edwardsiella tarda, and Aeromonas hydrophila (3).Plasmid variants appear to be relatively frequent in A. salmonicida subsp. salmonicida. The best example is the pRAS3 plasmid. To date, two variants of this plasmid (pRAS3.1 and pRAS3.2) have been described (8). The differences between them are very s...

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.