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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...

Aeromonas salmonicida , a bacterial fish pathogen, possesses a functional Type Three Secretion System (TTSS), which is essential for its virulence. The genes for this system are mainly located in a single region of the large pAsa5 plasmid. Bacteria lose the TTSS region from this plasmid through rearrangements when grown in stressful growth conditions. The A. salmonicida genome is rich in insertion sequences (ISs), which are mobile DNA elements that can cause DNA rearrangements in other bacterial species. pAsa5 possesses numerous ISs. Three IS 11 s from the IS 256 family encircle the rearranged regions. To confirm that these IS 11 s are involved in pAsa5 rearrangements, 26 strains derived from strain A449 and two Canadian isolates (01-B526 and 01-B516) with a pAsa5 rearrangement were tested using a PCR approach to determine whether the rearrangements were the result of an IS 11 -dependent process. Nine out of the 26 strains had a positive PCR result, suggesting that the rearrangement in these strains were IS-dependent. The PCR analysis showed that all the rearrangements in the A449-derived strains were IS 11 -dependent process while the rearrangements in 01-B526 and 01-B516 could only be partially coupled to the action of IS 11 . Unidentified elements that affect IS-dependent rearrangements may be present in 01-B526 and 01-B516. Our results suggested that pAsa5 rearrangements involve IS 11 . This is the first study showing that ISs are involved in plasmid instability in A. salmonicida .