Provisional epidemiological cutoff values for standard broth microdilution susceptibility testing of Flavobacterium columnare

Abstract

The gliding aquatic bacterium Flavobacterium columnare causes columnaris disease, a common problem for wild and farmed freshwater fish worldwide. Recently, a broth microdilution method was standardized to test the susceptibility of F. columnare against antimicrobials commonly used in aquaculture. We used this new method to measure the minimal inhibitory concentrations (MICs) of ten antimicrobials against 120 F. columnare isolates. The resulting MIC frequency distributions for each antimicrobial (1 MIC/isolate) were used to estimate epidemiological cut-off values (ECVs) which separate isolates with typical wild-type (WT) susceptibility from isolates with decreased non-wild-type (NWT) susceptibility. We identified 22 NWT isolates with elevated MICs relative to the ECV that covered 99.9% of the MIC distribution against one or more of the antimicrobials: ampicillin, enrofloxacin, erythromycin, florfenicol, flumequine, oxolinic acid or oxytetracycline. Ten of the NWT isolates had decreased susceptibility to a single antimicrobial class, six isolates to two antimicrobial classes and six isolates to three or more antimicrobial classes. The MIC frequency distributions and provisional cut-off values provide data needed to set epidemiological cut-off values to monitor for the development of antimicrobial resistance among F. columnare.

Pharmacokinetics and Tissue Distribution of Enrofloxacin Following Single Oral Administration in Yellow River Carp (Cyprinus carpio haematoperus)

“…And its MIC50 and MIC90 values were also observed as 0.016 and 0.125 µg/ml against F. columnare, respectively (47). However, another study determined its ECOFF against F. columnare as 0.03 µg/ml (48). All these MIC values were old [even from 1987; (47)] and not from the Yellow River carp.…”