Vancomycin MICs of the resistant mutants of S. aureus ATCC43300 vary based on the susceptibility test methods used

Abstract

Clinical failures with vancomycin against meticillin-resistant Staphylococcus aureus (MRSA) infections have challenged vancomycin's standing as a first-line antimicrobial for these infections. Conventional MIC tests were not predictive of the in vivo therapeutic effect of vancomycin. Thus, we tested the susceptibility for the resistant mutants in the mutant selection window of S. aureus ATCC43300 (a MRSA strain) by three different MIC-testing methods in this paper. The MIC of vancomycin was estimated at 2 μg ml⁻¹ on the Mueller-Hinton agar (MHA) plate only for the resistant mutant that was selected from the plate of vancomycin concentration 12 μg ml⁻¹. The obvious changes of susceptibility testing were found between the resistant mutants and S. aureus ATCC43300 on the Brain-Heart Infusion Agar (BHIA) plates. There were subtle changes in the MIC trend within the susceptible range with the result of Etest for the resistant mutants. The susceptibility for the subcultures of resistant mutants would fall back when the external drug environment disappeared. In comparison with the S. aureus ATCC43300, sequence analysis revealed that there were no mutations in the staphylococcal protein A (spa) sequencing of the resistant mutants. The spa tape is t421 for all isolates.