Wild-type cutoff values for standard broth microdilution antimicrobial susceptibility testing of Yersinia ruckeri isolates

Yersinia ruckeri is a Gram-negative bacterium that causes enteric redmouth disease. It commonly infects farmed salmonid fishes, often requiring antimicrobial treatment. Studies on antimicrobial susceptibility patterns of Y. ruckeri have found isolates with decreased susceptibility but have lacked internationally harmonized criteria, known as epidemiological cutoff values (ECVs), which provide consensus on identifying antimicrobial resistance. To address this need, we gathered minimal inhibitory concentration (MIC) testing data for 9 antimicrobials, generated at 22°C for 24-28 and/or 44-48 h using the standard broth microdilution testing method published by the Clinical and Laboratory Standards Institute (CLSI). The dataset includes MIC data from up to 431 isolates from 7 independent laboratories. Data for each antimicrobial were analyzed with the programs Normalized Resistance Interpretation (NRI) and ECOFFinder to compare the means and standard deviations for the 2 timepoints, and to calculate wild-type cutoff values. The parameters for the timepoints were very similar, which confirmed cutoffs were only needed for the 24-28 h incubation. The cutoff values for 8 of the 9 antimicrobials are potential ECVs that have been proposed to CLSI's Subcommittee on Veterinary Antimicrobial Susceptibility Testing. Calculated cutoffs for the final antimicrobial, erythromycin, were off-scale because the MIC values clustered at higher drug concentrations. The resulting new ECVs will be included in the next edition of the VET04 supplement, which will help clinicians and researchers advance antimicrobial resistance surveillance of this bacterium.