Occurrence of Transferable Integrons and sul and dfr Genes Among Sulfonamide-and/or Trimethoprim-Resistant Bacteria Isolated From Chilean Salmonid Farms

Salmon farming industry in Chile currently uses a significant quantity of antimicrobials to control bacterial pathologies. The main aims of this study were to investigate the presence of transferable sulfonamide- and trimethoprim-resistance genes, sul and dfr, and their association with integrons among bacteria associated to Chilean salmon farming. For this purpose, 91 Gram-negative strains resistant to sulfisoxazole and/or trimethoprim recovered from various sources of seven Chilean salmonid farms and mainly identified as belonging to the Pseudomonas genus (81.0%) were studied. Patterns of antimicrobial resistance of strains showed a high incidence of resistance to florfenicol (98.9%), erythromycin (95.6%), furazolidone (90.1%) and amoxicillin (98.0%), whereas strains exhibited minimum inhibitory concentrations (MIC(90)) values of sulfisoxazole and trimethoprim of >4,096 and >2,048 μg mL(−1), respectively. Strains were studied for their carriage of these genes by polymerase chain reaction, using specific primers, and 28 strains (30.8%) were found to carry at least one type of sul gene, mainly associated to a class 1 integron (17 strains), and identified by 16S rRNA gene sequencing as mainly belonging to the Pseudomonas genus (21 strains). Of these, 22 strains carried the sul1 gene, 3 strains carried the sul2 gene, and 3 strains carried both the sul1 and sul2 genes. Among these, 19 strains also carried the class 1 integron-integrase gene intI1, whereas the dfrA1, dfrA12 and dfrA14 genes were detected, mostly not inserted in the class 1 integron. Otherwise, the sul3 and intI2 genes were not found. In addition, the capability to transfer by conjugation these resistance determinants was evaluated in 22 selected strains, and sul and dfr genes were successfully transferred by 10 assayed strains, mainly mediated by a 10 kb plasmid, with a frequency of transfer of 1.4 × 10(−5) to 8.4 × 10(−3) transconjugant per recipient cell, and exhibiting a co-transference of resistance to florfenicol and oxytetracycline, currently the most used in Chilean salmon industry, suggesting an antibacterial co-selection phenomenon. This is the first report of the characterization and transferability of integrons as well as sul and dfr genes among bacteria associated to Chilean salmon farms, evidencing a relevant role of this environment as a reservoir of these genes.