Antimicrobial resistance patterns in Danish isolates of Flavobacterium psychrophilum

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The resistance pattern of Flavobacterium psychrophilum to the antimicrobial agents used in fish farming in Denmark was assessed in vitro using an agar dilution method. After identification of 387 isolates from clinical outbreaks of rainbow trout fry syndrome (RTFS) and the environment, the isolates were tested and the resulting antibiograms were used to predict the theoretical therapeutic efficacy and to evaluate if resistance had changed as a course of time. Antimicrobial agents included in this investigation were oxolinic acid (OXA), amoxicillin (AMX), potentiated sulfadiazine, oxytetracycline (OTC) and florfenicol (FLO). We found that F. psychrophilum isolates divided in susceptible and resistant clusters reflecting the reduced efficacy in practice when using OTC and AMX. The most recent isolates were less susceptible to AMX and OXA, whereas resistance to OTC seemed stable over the last 5 years. Apparently, F. psychrophilum carries intrinsic resistance towards the potentiated sulfonamides, and in correlation with this, we found very few susceptible isolates. All isolates were susceptible to FLO. The method used for determining minimum inhibitory concentration (MIC) follows the American guideline from The National Committee for Clinical Laboratory Standards (NCCLS) with some modifications to comply with the growth demand of this fastidious bacterium. We suggest future investigations use these guidelines as a benchmark.