Abstract
Despite their great importance for human therapy, quinolones are still used in Chilean salmon farming, with flumequine and oxolinic acid currently approved for use in this industry. The aim of this study was to improve our knowledge of the mechanisms conferring low susceptibility or resistance to quinolones among bacteria recovered from Chilean salmon farms. Sixty-five isolates exhibiting resistance, reduced susceptibility, or susceptibility to flumequine recovered from salmon farms were identified by their 16S rRNA genes, detecting a high predominance of species belonging to the Pseudomonas genus (52%). The minimum inhibitory concentrations (MIC) of flumequine in the absence and presence of the efflux pump inhibitor (EPI) Phe-Arg-β-naphthylamide and resistance patterns of isolates were determined by a microdilution broth and disk diffusion assays, respectively, observing MIC values ranging from 0.25 to >64 µg/mL and a high level of multi-resistance (96%), mostly showing resistance to florfenicol and oxytetracycline. Furthermore, mechanisms conferring low susceptibility to quinolones mediated by efflux pump activity, quinolone target mutations, or horizontally acquired resistance genes (qepA, oqxA, aac(6′)-lb-cr, qnr) were investigated. Among isolates exhibiting resistance to flumequine (≥16 µg/mL), the occurrence of chromosomal mutations in target protein GyrA appears to be unusual (three out of 15), contrasting with the high incidence of mutations in GyrB (14 out of 17). Bacterial isolates showing resistance or reduced susceptibility to quinolones mediated by efflux pumps appear to be highly prevalent (49 isolates, 75%), thus suggesting a major role of intrinsic resistance mediated by active efflux.
Highlights
The Chilean salmon farming industry is commonly affected by an important number of bacterial diseases causing high mortalities, prompting the necessity of using high amounts of antibiotics to ensure salmon production [1,2]
The main resistance mechanisms that additively contribute to quinolone resistance include one or a combination of target-site gene mutations that alter the drug-binding affinity of target enzymes, mutations that lead to reduced intracellular drug concentrations by either decreased uptake or increased efflux, and plasmid-encoded resistance genes that produce target protection proteins, drug-modifying enzymes, or multidrug efflux pumps [13,14,15,16]
Quinolone resistance associated with target protective enzymes mainly encoded by the qnr genes as well as the quinolone-modifying enzymes encoded by the aac(6 )-lb-cr gene and the quinolone efflux pumps encoded by the qepA and oqxAB genes, which are usually associated with plasmids, have been previously detected among bacteria isolated from fish farm-associated environments [22,23,24,25]
Summary
The Chilean salmon farming industry is commonly affected by an important number of bacterial diseases causing high mortalities, prompting the necessity of using high amounts of antibiotics to ensure salmon production [1,2]. Quinolones are intensively used in human therapy, they are still currently used in aquaculture [2,17,18], and the worldwide use of quinolones in the aquaculture industry has drastically decreased, the occurrence of bacteria resistant to this antibiotic group in fish farms has been previously detected [19,20,21]. Quinolone resistance associated with target protective enzymes mainly encoded by the qnr genes as well as the quinolone-modifying enzymes encoded by the aac(6 )-lb-cr gene and the quinolone efflux pumps encoded by the qepA and oqxAB genes, which are usually associated with plasmids, have been previously detected among bacteria isolated from fish farm-associated environments [22,23,24,25]. Quinolone resistance has been described in several pathogenic bacterial species [22,26,27,28], but it has never been found in fish pathogenic species isolated from diseased fish
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
