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.
