Vancomycin MIC Distribution among Methicillin-Resistant Staphylococcus Aureus . Is Reduced Vancomycin Susceptibility Related To MIC Creep?

AIM: To determine the distribution of vancomycin MIC and the frequency of S. aureus strains with reduced vancomycin susceptibility among Methicillin-Resistant Staphylococcus aureus (MRSA) isolates.

METHODS: MRSA isolates (n = 100) were tested for reduced susceptibility to vancomycin using MIC broth microdilution method (BMD), vancomycin screening agar with different vancomycin concentrations with and without casein, and Vitek 2 system.

RESULTS: BMD detected (22%) vancomycin-intermediate S. aureus (VISA) and (78%) vancomycin-susceptible S. aureus (VSSA) but couldn't detect nine (Heterogeneous VISA) (hVISA) isolates (9%) with MIC ≤ 2 µg/ml that grew on screening agar 4 µg/ml or 6 µg/ml. Adding casein to vancomycin screening agar increased detection rate of VISA by 4.5%. Screening agar with 6 µg/ml vancomycin overall detection rate for VISA was 95.45%. Probable 'pre-hVISA'isolates (17%) showed growth on vancomycin screening agar 2 µg/ml with casein. Vitek 2 system failed to detect any VISA isolates.

CONCLUSION: Vancomycin screening agar; 2 µg/ml and (4 and 6 µg/ml) were able to detect; probable "pre hVISA and (hVISA and VISA) isolates respectively based on their BMD MIC values. Decreased vancomycin susceptibility in MRSA isolates might be related to MIC creep. Analysis of vancomycin MIC values over longer periods is recommended to further study this phenomenon and its impact on vancomycin treatment failure.