Occurrence of florfenicol resistance in bacteria associated with two Chilean salmon farms with different history of antibacterial usage

Abstract

In Chile, the administration of florfenicol to control bacterial pathologies in freshwater salmon farms is a frequent practice, but no studies has been developed to evaluate the incidence of florfenicol-resistant bacteria in these cultures. This study was undertaken to obtain information on the occurrence of florfenicol-resistant bacteria in a farm (LF1) under florfenicol therapy two weeks before sampling date, located in the Rupanco Lake, and a farm (LF2) with no recent history of antibacterial therapy, located in the Llanquihue Lake. Samples from surface water, pellet, Salmo salar fingerlings and control and under-cage sediments were collected from each salmon farm. Total and florfenicol-resistant culturable counts were determined by a spread plate method, observing low (< 9%) proportions of florfenicol resistance in water and fingerling samples, whereas sediment samples from LF1 farm showed proportions ranging from 18.55 ± 2.85% (control) to 26.40 ± 1.56% (under-cage) and were significantly higher ( P < 0.05) than those from LF2 farm, with 0.29 ± 0.30% (control) and 0.69 ± 1.00% (under-cage) values, not observing a direct correlation with sediment organic enrichment. Also, unmedicated pelletized feed from LF1 farm showed high levels of resistance (34.79 ± 11.87%), and were significantly ( P < 0.05) higher than those from LF2 farm (< 0.001%). A number of 34 and 36 representative isolates recovered from LF1 and LF2 farms respectively, were selected and identified by using the Biolog Microbial ID System, observing that mainly belonged to Pseudomonas genus (67.65 and 66.67% in LF1 and LF2 farms) and the predominant species were P. putida and P. viridilivida (14.71%) in LF1 farm, and P. fluorescens (36.11%) in LF2 farm. Antibacterial susceptibility patterns of resistant strains from both Lakes were similar, showing important proportions of isolates resistance to ampicillin, erythromycin, furazolidone and cotrimoxazole and susceptibility to gentamicin, kanamycin and enrofloxacin. The minimum inhibitory concentration (MIC) of florfenicol against these selected bacteria were determined, observing MIC ranges of 256–> 1024 and 64–> 1024 μg ml − 1 for strains recovered from LF1 and LF2 farms, respectively. The important occurrence of florfenicol-resistant and antibacterial-multiresistant microorganisms in unpolluted and farm-impacted sites in Rupanco Lake suggests that use of florfenicol is not a necessary causal condition for the development of elevated frequencies of florfenicol resistance. Otherwise, considering that intrinsic resistance to many antibacterials has been extensively reported for Pseudomonas species, observed levels of resistance to florfenicol could be overestimated by the occurrence of innately resistant bacteria with multiple non-specific resistances. Our results prompt the necessity for maintaining a continuous surveillance of florfenicol resistance in salmon farms, as well as to characterize the molecular elements involved in these resistances to evaluate the feasibility of their dissemination among environmental microbiota.