Chloramphenicol and florfenicol susceptibility of fish-pathogenic bacteria isolated in France: comparison of minimum inhibitory concentration, using recommended provisory standards for fish bacteria

Aim: To investigate the distribution of antimicrobial resistance to phenicols in the fish pathogenic bacteria Aeromonas salmonicida, motile Aeromonas, Yersinia ruckeri, lactic bacteria and the nutritionally fastidious Flavobacterium psychrophilum. The last species was screened on two media (diluted Mueller-Hinton and peptone-enriched Anacker and Ordal), both supplemented with horse serum. Methods and Results: Minimal inhibitory concentration (MIC) assessment, using the agar dilution method according to proposed standards, confirmed that chloramphenicol resistance was more frequent and expressed at higher levels than florfenicol resistance. A significant resistant population, highlighted by the bimodal distribution of MICs, was detected only for chloramphenicol in A. salmonicida. No link could be found with the geographical origin of the isolates or fish species. Other cases of resistance appeared randomly distributed or related to the natural properties of the bacterial species. Although the two media used for testing F. psychrophilum resulted in comparable performances in dilution methods, Anacker and Ordal was more adapted to disc diffusion tests. Conclusion: Despite wide use, resistance to florfenicol does not seem to occur frequently in French fish farms. Significance and Impact of the Study: It is important to maintain a surveillance, as development of florfenicol resistance has occasionally been documented. For this purpose, and for the species studied in this work, the recently proposed standards appear generally well-adapted.