Detection of vancomycin MIC by agar dilution in clinical isolates of MRSA showing reduced zone of inhibition by disk diffusion method

Staphylococcus aureus is one of the most common causes of nosocomial infections, especially pneumonia, surgical site infections and blood stream infections and continues to be a major cause of community-acquired infections. Methicillin Resistant Staphylococcus aureus (MRSA) is resistant to the majority of antimicrobial agents available for clinical use, the glycopeptides vancomycin has been proposed as the drug of choice for treating such infections. The glycopeptide vancomycin was considered to be the best alternative for the treatment of multi drug resistant MRSA⁴. However, there are increasing numbers of reports indicating the emergence of vancomycin-resistant S. Aureus. Methicillin Resistant Staphylococcus aureus (MRSA) is resistant to the majority of antimicrobial agents available for clinical use, the glycopeptides vancomycin has been proposed as the drug of choice for treating such infections.

The present study was carried out to find out the presence of VISA and VRSA among 186 MRSA strains isolated from clinical specimens. Minimal inhibitory concentration (MIC) of vancomycin was determined by agar dilution method in Muller Hinton agar.

The MIC for 169 of 186 isolates (90.86%) for vancomycin was ≤2 mg/l indicating VSSA. 11(5.91%) isolates showed an MIC range between 4–8 mg/l, indicating VISA. Out of 11 VISA isolates the MIC for 6 (0.32%) isolates was in the range of >16mg/l indicating that VRSA.

The present study reveals the emergence of VRSA in tertiary care hospitals. Continuous efforts should be made to prevent the spread and emergence of glycopeptides resistance by early detection of the resistant strains and using proper infection control measures in hospital settings.