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 peptoneenriched 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.
These data suggest that clinical infections due to Pseudomonas aeruginosa are due, at least in part, to bacterial virulence. Furthermore, even within the constraints of small sample size, virulence of respiratory isolates is significantly correlated with the occurrence of P. aeruginosa bacteremia. Our results, as those published previously, suggest that at the strain level, acquisition of antimicrobial resistance is correlated with the diminution of virulence. However, at the population level, clinical isolates are more virulent as well as more frequently resistant to antibiotics, presumably due to acquisition of strains mostly through cross-contaminations.
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