Vancomycin and linezolid creeping minimum inhibitory concentrations in clinical isolates of MRSA: Prelude to overt resistance

• Author Details:   
• Sandhya Bhat ^*
• Desdemona Rasitha
• Ravichandran Kandasamy
• Sheela Devi Chandrakesan
• Reba Kanungo

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Background: Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) present a considerable problem in hospitals due to a lack of effective treatment options. The primary antimicrobial agents used to treat MRSA infections are vancomycin and linezolid; however, a rising trend of minimum inhibitory concentrations (MICs) for these antibiotics indicates a gradual decrease in susceptibility, which may result in treatment failure. This study aimed to determine the MIC values of vancomycin and linezolid in MRSA clinical isolates to detect any creep toward resistance.

Materials and Methods: This study included 190 MRSA isolates to determine MICs for oxacillin, vancomycin, and linezolid using the Epsilometer test, adhering to the Clinical and Laboratory Standards Institute (CLSI) 2022 guidelines. The correlation between oxacillin, vancomycin, and linezolid MICs was estimated using Spearman's rho correlation coefficient.

Results: In the present study, MRSA prevalence was 33.8% (n=562/190). Males were outnumbered, and most patients (n=83; 43.7%) belonged to the 31-50 age group. The oxacillin MIC values ranged from 0.75 to ?256 ?g/mL, vancomycin MICs from 0.38 to 2?g/mL, and linezolid MICs from 0.38 to 4?g/mL among MRSA clinical isolates. A gradual increase in vancomycin and linezolid MICs was documented. The oxacillin and vancomycin MICs showed a moderate correlation (0.666), while the oxacillin and linezolid MIC values showed no correlation with the vancomycin and linezolid MIC values.

Conclusion: The gradual increase in MICs for vancomycin and linezolid in MRSA isolates suggests a trend that could lead to total resistance. Ongoing surveillance of MIC values is crucial for enabling prompt adjustments in therapy and preventing treatment failures.

Keywords: Minimum inhibitory concentration, Epsilometer test, Vancomycin, Linezolid, Cefoxitin disk diffusion.