Prevalence and genotypic relatedness of methicillin resistant Staphylococcus aureus in a tertiary care hospital

Methicillin-resistant Staphylococcus aureus (MRSA) is the most common multidrug-resistant pathogen causing nosocomial infections across the world. MRSA is not only associated with significant mortality and morbidity but also places a large economic strain on our health care system. MRSA isolates are also typically resistant to multiple, non-β-lactam antibiotics. We conducted a prospective study in a tertiary care hospital, to determine the prevalence of MRSA and to establish the clonal distribution of MRSA isolates recovered from various clinical specimens.

Clinical samples were cultured and S. aureus was identified as per standard microbiological procedures. Susceptibility testing was done by agar disk diffusion and minimum inhibitory concentration (MIC) method as recommended by CLSI. Methicillin resistance was detected by phenotypic methods namely, oxacillin disc diffusion (ODD), minimum inhibitory concentration (MIC) of oxacillin, cefoxitin disk diffusion (CDD), and MIC of cefoxitin. Amplification of mecA gene by PCR was used as gold standard for detection of methicillin resistance. Pulsed field gel electrophoresis (PFGE) typing was performed for MRSA isolates.

Out of 390 S. aureus isolates, 154 (39.48%) isolates were MRSA and 236 (60.51%) isolates were MSSA. Penicillin was the least effective antibacterial drug against the hospital associated S. aureus isolates with 85.64% resistance rate. All the isolates were susceptible to vancomycin. The MRSA showed a high level of resistance to all antimicrobials in general in comparison to the MSSA and the difference was statistically significant (P < 0.05). Multiplex PCR performed for all strains showed amplification of both the mecA and nucA genes in MRSA strains whereas MSSA strains showed amplification of only nucA gene. PFGE of these isolates showed 10 different patterns.

Prevalence of MRSA in our hospital was 39.48%. Most of these isolates were resistant to erythromycin, clindamycin, cotrimoxazole, and ciprofloxacin, whereas high sensitivity was seen to vancomycin followed by gentamicin. CDD and MIC for cefoxitin showed 100% sensitivity, specificity, PPV and NPV as compared to PCR for mecA gene. In maximum number of isolates PFGE type A pattern was seen suggesting clonal relatedness.