Antimicrobial susceptibility of Flavobacterium psychrophilum isolates from Ontario

Flavobacterium psychrophilum is the etiological agent of bacterial coldwater disease (BCWD), an important disease in the Ontario fish farming industry and in finfish aquaculture in temperate waters worldwide. The development of antimicrobial resistance by F. psychrophilum is a concern because management of outbreaks of BCWD often requires the use of antibiotics. Seventy-two isolates of F. psychrophilum collected over a 16-year period from farmed salmonids with clinical signs of BCWD were tested for susceptibility to 10 antimicrobial agents using cation-adjusted Mueller-Hinton broth in custom Trek Sensititre susceptibility plates for aquaculture. The minimum inhibitory concentrations (MICs) for the isolates were determined by means of a broth microdilution antimicrobial susceptibility testing method established by the Clinical and Laboratory Standards Institute. Most of the F. psychrophilum isolates had decreased susceptibility to two of the four antibiotics licensed for use in Ontario (i.e., ormetoprim-sulfadimethoxine [> or =0.5/9.5 .tg/mL for 93% of isolates] and trimethoprim-sulfamethoxazole [> or = 0.25/4.8 microg/mL for 89% of isolates]). High MIC values (> or =2 microg/mL) were obtained for florfenicol and oxytetracycline in 53% and 61% of the isolates, respectively, and 83% of the isolates were relatively susceptible (< or =16 microg/mL) to erythromycin. The MIC values were also high for ampicillin, oxolinic acid, and gentamicin.