Abstract

Florfenicol is an important antibiotic in veterinary medicine that is used extensively in aquaculture, including salmon farming in Chile. We analysed a set of 119 florfenicol-resistant Gram-negative bacilli from seven freshwater Chilean salmon farms for the molecular determinants involved in the florfenicol resistance. Ninety-seven of these strains were glucose non-fermenting bacilli, mainly belonging to the Pseudomonas genus, whereas 22 strains were glucose-fermenters. The floR gene was detected in 26 strains (21.8%) that had been isolated from three of the seven salmon farms. Most of the floR-carrying strains were glucose fermenters (21 strains), and most of the floR-carrying strains were also resistant to streptomycin, chloramphenicol and oxytetracycline. The minimum inhibitory concentrations against florfenicol were assessed in the presence and absence of the efflux pump inhibitor Phe-Arg-β-naphthylamide (MC-207,110). There was evidence that in the majority of non-fermenting bacteria (82 strains), florfenicol resistance was at least partially mediated by non-specific efflux pump systems. Given the diversity of antibiotic resistance patterns observed in this study in the floR-positive isolates, a single antibiotic has the potential to co-select for a diversity of resistances. For this reason, human health as well as animal health can potentially be impacted by the use of antibiotics in aquaculture. To assess this potential risk, future studies should focus on the ability of different antibiotics used in aquatic environments to co-select for multiple resistances, the molecular basis of this diversity of resistance, and whether the genes conferring resistance can be transferred to other bacteria, including those of human health concern. © 2009 Blackwell Verlag GmbH.