Establishing tentative cut-off values for disk diffusion method for Pasteurella multocida and Mannheimia haemolytica from livestock animals in Japan.

Although the disk diffusion method is a useful and widely utilized antimicrobial susceptibility testing method in clinical veterinary practice, the available clinical breakpoints (CBPs) for the combination of bacterial species and antimicrobials are still limited. This study aimed to establish tentative cut-off values by analyzing the relationship between minimum inhibitory concentration and the inhibition zone diameter of the disk diffusion method for Pasteurella multocida and Mannheimia haemolytica, collected by the Japanese Veterinary Antimicrobial Resistance Monitoring system (JVARM), for the antimicrobial agents approved in Japan for the treatment of respiratory diseases in cattle and swine. Among the antimicrobial agents without CBPs for the disk diffusion method in the Clinical and Laboratory Standards Institute (CLSI), we were able to determine the tentative cut-off values of ampicillin (ABPC, S ≥25 mm, R ≤24 mm in swine), dihydrostreptomycin (DSM, S ≥8 mm in cattle, S ≥12 mm in swine), kanamycin (KM, S ≥16 mm in cattle, S ≥17 mm in swine), tetracycline (TC, S ≥21 mm, R ≤20 mm in cattle) and tiamulin (TML, S ≥13 mm in swine) for P. multocida, and CEZ (S ≥23 mm), DSM (S ≥10 mm), KM (S ≥18 mm) and TC (S ≥23 mm, R ≤20 mm) for M. haemolytica. These results should encourage the practical use of antimicrobial susceptibility testing in the clinical field and contribute to furthering prudent use of antimicrobial agents.