Epidemiological Cutoff Values for Standard Broth Microdilution and Disk Diffusion Susceptibility Testing of Aeromonas hydrophila Isolated from Fish

Abstract

Aeromonas hydrophila and other closely related Aeromonas species cause motile aeromonad septicemia, a common fish disease. The disease affects many aquaculture sectors potentially requiring antimicrobial treatments. Therefore, researchers and laboratory diagnosticians need criteria called epidemiological cutoff values (ECVs) to determine whether a bacterial isolate has developed decreased susceptibility to an antimicrobial. To generate ECVs for this bacterium, we assembled a diverse collection of 245 isolates previously identified as A. hydrophila from fish. Using rpoD sequencing, we confirmed that 97 of the 245 isolates were A. hydrophila. We allocated the isolates among three laboratories and tested their susceptibility against eight antimicrobials using standard Clinical and Laboratory Standards Institute (CLSI) disk diffusion and broth microdilution methods. The resulting frequency distributions were statistically analyzed to determine wild-type cutoff estimates, which, along with scatterplots, were used to estimate potential ECVs. In collaboration with the CLSI, aquaculture working group, we proposed ECVs for six of the eight antimicrobials tested. Subsequently, the CLSI Subcommittee on Veterinary Antimicrobial Susceptibility Testing reviewed our data and approved the ECVs to be added to the 2020 edition of the VET04 performance standards for antimicrobial susceptibility testing of aquatic bacteria.

Keywords: Aeromonas hydrophila; epidemiological cutoff values; fish; motile aeromonad septicemia.

FIG. 1.

Scatter plots of ZOI data versus MIC data from standard testing of Aeromonas hydrophila isolates (n = 104) against (A) erythromycin, (B) florfenicol, and (C) gentamicin. The dashed lines denote the CLSI-approved ECV listed in Table 4. Gray shaded areas highlight isolates falsely characterized as WT or NWT because results from the testing methods did not agree. There are no shaded areas for erythromycin since a ZOI cutoff could not be determined. BMD, broth microdilution; CLSI, Clinical and Laboratory Standards Institute; DD, disk diffusion; ECV, epidemiological cutoff value; MICs, minimal inhibitory concentrations; NWT, nonwild-type; WT, wild-type; ZOI, zone of inhibition.

FIG. 2.

Scatter plots of ZOI data versus MIC data from standard testing of Aeromonas hydrophila isolates (n = 104) against (A) enrofloxacin, (B) oxolinic acid, and (C) oxytetracycline. The dashed lines denote the CLSI-approved ECV listed in Table 4. Gray shaded areas highlight isolates falsely characterized as WT or NWT because results from the testing methods did not agree.