Comparison of penicillin-G susceptibility testing methods of staphylococci isolated from bovine mastitis

Penicillin-G susceptibility was analyzed on forty staphylococcal strains isolated from mastitic bovine quarters (29 coagulase positive and 11 coagulase negative) by the standard Kirby-Bauer agar diffusion method, the epsilon-Test, the Vetmic, turbidimetric MIC analysis in Iso-Sensitest Broth (ISB) and whey. Penicillinase production was tested. Parallel susceptibility tests were carried out in whole milk, whey and ISB using resazurin and triphenyltetrazolium as the indicators of bacterial activity. The traditional susceptibility testing methods (radial agar diffusion, MIC in broth culture) showed good agreement with each other and confirmed that the tests can be used interchangeably with the current breakpoint values (0.25 micrograms/ml and phi 26 mm). The tests carried out in whey showed good correlation with the traditional tests. However, the susceptibility testings in milk resulted in additional variation. Therefore, traditional susceptibility tests of Penicillin-G in artificial media are limited in estimation of bacterial susceptibility when they grow in whole milk. The relevance of this observation regarding mastitis therapy is discussed.