In vitro vancomycin susceptibility amongst methicillin resistant Staphylococcus aureus

Area of Science:

• Microbiology
• Infectious Diseases
• Clinical Pharmacy

Background:

• Vancomycin is the primary treatment for Methicillin-Resistant Staphylococcus aureus (MRSA) infections.
• Increasing prevalence of Staphylococcus aureus with reduced vancomycin susceptibility (SA-RVS) necessitates reliable detection methods.
• Current guidelines rely on Minimum Inhibitory Concentration (MIC) via broth or agar dilution, with the Epsilometer (E) test as an alternative.

Purpose of the Study:

• To determine the prevalence of SA-RVS in clinical MRSA isolates.
• To compare the accuracy of vancomycin MIC determination using agar dilution versus the E test.
• To evaluate the E test as a screening tool for vancomycin susceptibility.

Main Methods:

• A prospective study involving 232 clinical MRSA isolates from a tertiary care hospital.
• Vancomycin MIC was determined for all isolates using both agar dilution and E test methods.
• Linezolid susceptibility was also assessed for all isolates.

Main Results:

• All 232 MRSA isolates demonstrated susceptibility to Linezolid.
• Two isolates exhibited vancomycin MIC ≥4 μg/ml.
• Agar dilution yielded vancomycin MIC50 and MIC90 values of 0.5 and 0.2 μg/ml, respectively.
• The E test showed over 93.5% agreement with agar dilution for vancomycin susceptibility.
• E test demonstrated a sensitivity of 1.0 and a positive predictive value of 0.5 compared to agar dilution.

Conclusions:

• MRSA isolates in this study remained susceptible to vancomycin and Linezolid.
• The E test is a suitable and comparable method for initial screening of vancomycin susceptibility in MRSA.
• Ongoing surveillance of SA-RVS is crucial due to potential geographic variations in prevalence.