Vancomycin MIC creep and the emergence of oxacillin-susceptible MRSA: A three-year surveillance study from a tertiary care centre.

INTRODUCTION

Methicillin resistant S. aureus (MRSA) remains a high-priority pathogen due to its multidrug resistance. Vancomycin remains mainstay for treating serious MRSA infections. Gradual upward shift of MICs among vancomycin susceptible S. aureus termed as "MIC creep" and emergence of oxacillin-susceptible MRSA (OS-MRSA) pose therapeutic and diagnostic challenges. This study aimed to investigate trends of vancomycin susceptibility and OS-MRSA among S. aureus isolates during the period 2021-2023.

METHODS

A total of 1626 non-duplicate clinical isolates of S. aureus were characterized. Vancomycin MIC and phenotypic detection of MRSA was done by Vitek-2. MRSA were characterized as OS-MRSA and oxacillin resistant MRSA (OR-MRSA) based on MIC. OS-MRSA was confirmed by mecA/mecC gene. Vancomycin agar screen plates (3 and 6 μg/mL) were used for VISA detection. Vancomycin consumption was calculated as DDD/100 patient days.

RESULTS

During 2021-2023, prevalence of MRSA increased from 65.4% to 73.9% and OS-MRSA cases doubled. All OS-MRSA were mecA-positive. Vancomycin MICs remained within the susceptible range (≤2 μg/mL) but significant increase in geometric mean MIC and increase in isolates (22.6% to 44.1%, P = 0.01) with MIC ≤2 μg/mL especially among OR-MRSA was observed. Vancomycin consumption increased by a 183% (2021-2024). OS-MRSA displayed greater sensitivity to non-glycopeptide antibiotics compared to OR-MRSA.

CONCLUSION

OS-MRSA poses diagnostic challenges and adds complexity to MRSA epidemiology. The upward trend of vancomycin MIC is suggestive of "MIC creep," which may reflect early shifts in susceptibility under antimicrobial pressure. Continuous surveillance of resistance phenotypes and antimicrobial stewardship is essential to guide effective management of S. aureus infections.