Oxacillin, cephalothin, and vancomycin tube macrodilution MBC result reproducibility and equivalence to MIC results for methicillin-susceptible and reputedly tolerant Staphylococcus aureus isolates.

Measurement of antimicrobial killing endpoints of Staphylococcus aureus isolates in tube macrodilution MBC testing has been difficult because of multiple technical factors. A total of 41 fresh clinical isolates and 23 reputedly oxacillin-tolerant strains were examined by a modification of the Taylor MBC method. Oxacillin, cephalothin, and vancomycin MBCs were equal to MICs for most strains and were seldom more than fourfold greater than the corresponding MICs after a 48-h incubation. Oxacillin MBC result reproducibility for S. aureus ATCC 25923 and clinical isolates was better than that of cephalothin and vancomycin, and reproducibility improved after a 48-h incubation. Measurement of the percentage of the initial inoculum remaining after 24 and 48 h of incubation for the strains for which the MBCs were highest confirmed improved killing over a wide range of antimicrobial concentrations after a 48-h incubation. Since S. aureus MBC testing is expensive, is subject to error, and almost always gives results equal to the MIC, we suggest that MBC testing is an experimental reference laboratory test that should not be done by clinical microbiology laboratories. Antimicrobial selection should be based on reproducible and standardized MIC tests.