VANCOMYCIN SUSCEPTIBILITY STATUS AMONG CLINICAL MRSA ISOLATES AND DETECTION OF VISA, hVISA IN A TERTIARY CARE HOSPITAL

Abstract

BACKGROUND In the present scenario due to indiscriminate use of many antibiotics including vancomycin Staphylococcus aureus (S. aureus) demonstrates a reduced susceptibility to it. Reduced susceptibility to vancomycin is one of the bigger problems in the treatment of infections caused by S. aureus. METHODS To detect reduced susceptibility for vancomycin, vancomycin disk diffusion by Kirby-Bauer disk diffusion method, vancomycin agar screen by Hiramatsu et al (4 µg/ml of vancomycin) and CDC/CLSI (6 µg/ml of vancomycin) method and vancomycin minimum inhibitory concentration (MIC) were performed. MIC's were determined by broth microdilution as well as ETEST. Simplified Population Analysis Profile (SPAP) was also performed for confirmation of h-VISA isolates. RESULTS MIC's of 165 isolates was ≤2μg/ml by ETEST while 35 isolates had MIC's >2µg/ml. Among these 35 isolates, two isolates had MIC of 4µg/ml and one had MIC of 6µg/ml and therefore, these three isolates may have been VISA as per CLSI guidelines. Using broth microdilution, all the isolates had MIC's of ≤2μg/ml. Vancomycin agar screen was performed by cut using method followed by Hiramatsu et al (4 µg/ml of vancomycin) and CDC/CLSI (6µg/ml of vancomycin) method. By the former method, four isolates demonstrated growth at 4 µg/ml and 196 isolates had no growth. These four isolates were further confirmed by Simplified Population Analysis Profile (SPAP) and were confirmed as h-VISA (Heterogeneous-Vancomycin Intermediate Staphylococcus aureus) isolates with MIC's 8 µg/ml. Using CDC/CLSI method, all 200 isolates did not grow at 6 µg/ml of vancomycin. CONCLUSIONS Detection of hVISA hetero-resistance phenotype is very difficult. The best method for detecting hVISA is controversial. Population Analysis Profile (PAP) is considered as the gold standard, but this method is time consuming, expensive and labour intensive. So, it can't be routinely used in a large busy setup.