Approved consensus-driven epidemiological cutoff values for standard broth microdilution antimicrobial susceptibility testing of aquatic Vibrio harveyi isolates.

Judicious use of antimicrobials in aquaculture requires reliable antimicrobial susceptibility testing (AST) of bacterial pathogens for resistance surveillance and for advising therapy decisions. To improve AST of aquatic bacterial pathogens such as Vibrio harveyi, the Clinical and Laboratory Standards Institute (CLSI) has standardized methods specific to testing isolates collected from fish and other aquatic animals. However, no criteria, called epidemiological cutoff values (ECVs), exist yet to interpret results when testing V. harveyi with these standard methods. Microbiologists use ECVs to determine whether an isolate has decreased susceptibility to an antimicrobial relative to other isolates of the same bacterium. In this study, we generated minimal inhibitory concentration (MIC) data using 3 independent laboratories that tested 76 isolates with the CLSI standard broth microdilution method at 28°C for 24-28 h against 9 antimicrobials. The resulting MIC data for 6 of the antimicrobials listed below was combined with previously published data (Smith et al. 2023; Dis Aquat Org 155:35-42) and analyzed with the programs Normalized Resistance Interpretation (NRI) and ECOFFinder to calculate potential ECVs. In collaboration with CLSI's Working Group on Aquatic Animals, the potential ECVs were proposed to CLSI's Subcommittee on Veterinary Antimicrobial Susceptibility Testing, which voted to accept the values. These new ECVs will be included in the next edition of the VET04 supplement. The approved ECVs for enrofloxacin, florfenicol, gentamicin, oxolinic acid, oxytetracycline, and trimethoprim/sulfamethoxazole are ≤1, ≤2, ≤4, ≤1, ≤1, and ≤0.5/9.5 µg ml-1, respectively. These new interpretive criteria will improve international harmonization regarding monitoring efforts on antimicrobial susceptibility of V. harveyi.