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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.

frontiers in microbiology

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