Sub‐Inhibitory Concentrations of Florfenicol Modulate the Expression of Biofilm Formation and Antibiotic Resistance‐Associated Genes in Biofilm‐Embedded <i>Piscirickettsia salmonis</i>

Piscirickettsiosis is the most prevalent bacterial disease affecting Chilean aquaculture and responsible for the majority of mortality in salmonids. Currently, large quantities of antibiotics, predominantly florfenicol, are used in the Chilean aquaculture industry, and sub-MIC concentrations of this antibiotic, similar to what occurs in the marine environment, have been shown to induce biofilm formation on both biotic and abiotic surfaces when sub-MIC doses of florfenicol, raising concerns about the emergence of antibiotic-resistant bacterial strains. Thus, the aim of this study was to evaluate whether in vitro sub-MIC concentrations of florfenicol induce the expression of genes associated with biofilm formation and antibiotic resistance in the biofilm-embedded P. salmonis. Interestingly, in vitro analyses showed that sub-MIC dilutions of antibiotic significantly modulated the expression of an efflux pump acrAB and the two-component systems cpxAR, and qseBC, as well as the antibiotic resistance-associated genes tclor/tflor and t.flor in the biofilm-embedded P. salmonis isolates tested. Thus, this study highlights the negative consequences of the extensive use of antibiotics in aquaculture, which can promote biofilm formation in marine bacterial pathogens, potentially facilitating the spread of resistance genes among different bacterial species in the aquatic environment and increasing the risk of reinfection within culture systems.