In-vitro susceptibility of Methicillin resistant Staphylococcus aureus isolates to linezolid at a tertiary care hospital of Karachi, Pakistan

Objective: To determine frequency of methicillin resistant Staphylococcus aureus (MRSA) and its susceptibility to linezolid in specimens collected from inpatients. Methodology: This descriptive cross sectional study was conducted over a period of eleven months from March, 2012 to Feb, 2013 and was carried out in the department of Microbiology, Ziauddin Medical University, Karachi, Pakistan. A purposive sampling of 303 clinical isolates which includes pus, blood, sputum, tracheal aspirate, sterile body fluids, HVS and CSF were inoculated and identified using standard microbiological technique. Antimicrobial susceptibility testing of cefoxitin (30ug) and linezolid (30ug) was performed on Muller Hinton agar (MHA) by standard Kirby Bauer's disk diffusion method according to the Clinical Laboratory Standard Institute (CLSI) criteria 2012. S. aureus was considered MRSA when zone of inhibition of cefoxitin disk was equal to 21 mm or less. Results: A total of 303 S. aureus isolates were collected and processed. The prevalence of MRSA was found to be 44.9%. Linezolid was 100% sensitive in all MRSAisolates. Conclusion: High prevalence of MRSA was observed in our institute. Our data underlines the need for surveillance, typing and constant reassessment of existing strategies to treat and control MRSA infections. All strains were susceptible to linezolid, which may be used for empirical therapy against MRSA infections. (Rawal Med J 201 ;43:502-506).