Wild-type cutoff values for standard broth microdilution antimicrobial susceptibility testing of Edwardsiella ictaluri and E. piscicida isolates

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Edwardsiella ictaluri and E. piscicida are bacterial pathogens that infect a variety of commercially important wild and cultured fish and cause significant economic losses among various farmed fish sectors, especially the catfish industries in the USA and Vietnam. The Clinical and Laboratory Standards Institute (CLSI) offers standard antimicrobial susceptibility testing (AST) methods for these pathogens and others, but internationally harmonized criteria, called epidemiological cutoff values (ECVs), are still needed to identify isolates with decreased antimicrobial susceptibility. To address this need, minimal inhibitory concentration (MIC) data for 9 antimicrobials were generated using the standard broth microdilution AST method at 28 ± 2°C for 44-48 h. The data sets comprise MICs from 101 E. ictaluri isolates from 4 independent laboratories and 115 E. piscicida isolates from 3 laboratories. Aggregated MIC distributions were used to calculate wild-type (WT) cutoff values for the 18 bacteria/antimicrobial combinations. WT cutoff values for E. ictaluri against ampicillin, enrofloxacin, erythromycin, florfenicol, gentamicin, oxolinic acid, oxytetracycline, ormetoprim/sulfadimethoxine, and trimethoprim/sulfamethoxazole were ≤1, ≤0.12, ≤128, ≤2, ≤2, ≤0.5, ≤2, ≤0.5/9.5, and ≤0.25/4.8 µg ml-1, respectively. WT cutoff values for E. piscicida were ≤8, ≤0.12, n/a, ≤2, ≤2, ≤0.25, ≤2, ≤0.12/2.4, and ≤0.12/2.4 µg ml-1, respectively. No E. piscicida WT cutoff value could be calculated for erythromycin. These WT cutoff values provided the CLSI with data needed to approve ECVs which will be included in the next edition of the VET04 supplement. CLSI-approved ECVs resulting from this work will greatly improve antimicrobial resistance surveillance of these bacterial pathogens impacting global aquaculture.

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