Multidrug and Methicillin Resistant Staphylococcus aureus (MRSA) Isolated from Clinical Samples

Aim: The present study aimed to detect inducible clindamycin resistance, vancomycin resistance and mupirocin resistance among MRSA isolates.

Background: Multidrug and methicillin resistant Staphylococcus aureus is a widely spread problem in clinical environment, therefore current information regarding antimicrobial susceptibility of the pathogen is very important for the treatment of patients and in control and prevention strategy. S. aureus, a common pathogen, well known for its multidrug resistance. Existence of MRSA is further worsened by inducible clindamycin resistance and emerging glycopeptide resistance.

Material and methods: The study was conducted for a period of 6 months from May to October 2010. A total of 100 non-repetitive S. aureus isolates from various clinical specimens were included in the study. A total of one hundred non-repetitive isolates were subjected to routine antibiotic susceptibility testing by Kirby Bauer’s disc diffusion method including cefoxitin disc for MRSA. Inducible clindamycin resistance was detected by D-test, E-test for vancomycin MIC and mupirocin resistance by disc diffusion.

Results: Twenty three (85.2%) isolates showed inducible clindamycin resistance, one (3.7%) showed constitutive resistance and three (11.1%) showed MS phenotypes. Inducible clindamycin resistance (35.7%), constitutive resistance (2.3%) and MS phenotype (7.1%) were found to be higher in MRSA as compared to MSSA. Only one isolate with vancomycin MIC 4µg/ml by E-test was considered as VISA. In our study, only one strain, which had MIC 4µg/ml has been considered as VISA. VISA may demonstrate heteroresistance or there may be subpopulations that are resistant. Screening for hVISA requires additional testing to reveal its hetero-variant phenotype and these methods are more labor intensive and costly than routine susceptibility testing. Our study detected mupirocin resistance in 11(26.1%) MRSA and 30(51.72%)MSSA isolates, which is a cause for concern. Study showed that D-test should be included as a routine disc diffusion test to prevent therapeutic failure with clindamycin.