Verification of a simplified disk diffusion method for antimicrobial susceptibility testing of bovine mastitis isolates

Bovine mastitis is mainly treated with antimicrobials. Determination of antimicrobial treatments based on the results of an antimicrobial susceptibility test is important to reduce the risk of emergence of antimicrobial resistance and to provide effective treatment. In Japan, not only the standardized agar disk diffusion method (standardized-ADD) based on Clinical and Laboratory Standards Institute guidelines, but also the agar disk diffusion method further simplified (simplified-ADD) are widely used as antimicrobial susceptibility tests for bovine mastitis isolates in the clinical laboratory. However, whether the simplified-ADD is a useful alternative to the standardized-ADD has not yet been sufficiently verified. Therefore, to verify the usefulness of the simplified-ADD, we compared the results of the standardized-ADD and the simplified-ADD using clinical isolates of bovine mastitis. Following testing of 83 isolates from 11 bacterial species, the correlation coefficient of the disk zone diameters in both methods was 0.92, indicating that the simplified-ADD is effective as an alternative method to the standardized-ADD. However, because the disk zone diameter tended to be smaller in the simplified-ADD than in the standardized-ADD, sufficient attention should be paid to this point when determining the treatment for clinical cases of mastitis from the results of the simplified-ADD. The fact that a difference in the results between the two methods was present means that the results cannot be interpreted based on the same criteria. Therefore, determination of the criteria appropriate for the simplified-ADD is needed.