Florfenicol at subinhibitory doses activates biofilm and resistance genes in Piscirickettsia
Scientists have shown that subinhibitory concentrations of florfenicol not only induce biofilm formation, but also modulate genes associated with resistance, favouring the persistence of the pathogen.
Published
Many fish pathogens produce biofilms in response to various stressors, such as antibiotics. These biofilms provide several survival advantages and can act as a reservoir of pathogens in the marine environment, as have been studied in Picirickettsia salmonis, the bacterium that causes SRS (salmon rickettsial septicaemia).
According to the latest report from Chile's fisheries and aquaculture agency Sernapesca on the use of antimicrobials and antiparasitics in salmon farming, during the first half of 2025 florfenicol was the main antimicrobial used in farms (96.8%), mainly associated with the control of SRS (94.7% of use), and to a lesser extent for the treatment of BKD (bacterial kidney disease) and tenacibaculosis.
It is now known that exposing P. salmonis to minimal subinhibitory (sub-MIC) concentrations of florfenicol, similar to those found in the marine environment, induces biofilm formation on biotic and abiotic surfaces, "raising concerns about the emergence of antibiotic-resistant bacterial strains", the authors of a new study noted.
Significant modulation
In the research by scientists from the Austral University of Chile, Universidad Mayor, Universidad de Concepción, Universidad Andrés Bello, Berking Biotechnology and the Incar Center, the experts evaluated whether sub-MIC concentrations of florfenicol in vitro induces the expression of genes associated with biofilm formation and antibiotic resistance in P. salmonis.
"Interestingly, in vitro analyses showed that subinhibitory dilutions of the antibiotic significantly modulated the expression of the 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 P. salmonis isolates embedded in the analyzed biofilms,” the experts explained.
According to the scientists, these results highlight the negative consequences of the extensive use of antibiotics in aquaculture, "which can promote the formation of biofilms in marine bacterial pathogens, potentially facilitating the spread of resistance genes between different bacterial species in the aquatic environment and increasing the risk of reinfection in farming systems".
Read the abstract of the study titled “Sub-Inhibitory Concentrations of Florfenicol Modulate the Expression of Biofilm Formation and Antibiotic Resistance-Associated Genes in Biofilm-Embedded Piscirickettsia salmonis” here.
