Rising incidence of high MIC for vancomycin among Staphylococcus aureus strains at a tertiary care hospital in South India.

Sir,

The growing prevalence of methicillin-resistant Staphylococcus aureus (MRSA) as a cause of infections both in the hospital and the community led to increased use of the glycopeptide antibiotic vancomycin over the past three decades.[1] As a consequence, selective pressure was established that eventually resulted in the emergence of strains of S aureus with decreased susceptibility to vancomycin and other glycopeptides. The reports of vancomycin-intermediate S aureus (VISA) and vancomycin-resistant S aureus (VRSA) has been increasing from various parts of the world. The first clinical isolate of VRSA was reported from the United States in 2002.[2] More recently, some workers have reported vancomycin-resistant staphylococcal strains from Jordan[3] and India.[4]

VISA and VRSA strains are not detected by the disk diffusion method. Acceptable methods used to detect these strains are nonautomated and include broth or agar dilution and the E-test. Also, the Clinical Laboratory Standards Institute (CLSI) has recently lowered breakpoints for vancomycin and, presently, strains with minimum inhibitory concentration (MIC) of 4–8 μg/ml are considered VISA and with MIC≥16 μg/ml are considered VRSA.[5]

In our study, S aureus isolates from various clinical specimens like pus, blood, catheter tips, and urine were included. All S aureus isolates were subjected to susceptibility testing by the Kirby-Bauer disk diffusion method, and those isolates (n=80) showing a diminished zone of inhibition for vancomycin were subjected to MIC testing of vancomycin by the agar dilution method. The tests were performed according to CLSI guidelines.[5]

Out of 80 samples, 75 samples had MIC≤2 μg/ml (VSSA), 4 samples had MIC 4–8 μg/ml (VISA), and 1 strain was VRSA (MIC>16 μg/ml). Sensitivity to ciprofloxacin, erythromycin, gentamycin, and linezolid among the isolates was 57%, 68%, 41%, and 100%, respectively. The increasing MICs of vancomycin in S. aureus isolates should ring an alarm bell for prescribers as strains with reduced susceptibility could be the harbinger of future strains with full-blown resistance. In view of the limited therapeutic options for the treatment of MRSA infections, prudent use of vancomycin, continuous surveillance for VISA and VRSA strains, and appropriate infection control practices for the prevention of spread of such strains in the hospital environment are strongly recommended.