Minimum inhibitory concentration of vancomycin to methicillin resistant Staphylococcus aureus at a tertiary care hospital in Sri Lanka

Background: Methicillin Resistant Staphylococcus aureus (MRSA) has evolved as a serious threat to public health. Objectives: The objectives of the present study were to determine the antibiotics susceptibility patterns of S. aureus and the minimum inhibitory concentration (MIC) of vancomycin to MRSA isolated from different clinical samples at the Colombo South Teaching Hospital (CSTH) in Sri Lanka. Methodology: A total 72 isolates of S. aureus, obtained from different clinical samples at the CSTH, from January to May 2017 were included in the study. S. aureus isolates were identified by Gram stain, colony morphology, catalase, slide/tube coagulase tests. The antibiotic susceptibility tests were carried as per Clinical Laboratory Standards Institute (CLSI) guidelines. MRSA isolates were detected using the cefoxitin (30 μg) disk diffusion test. Inducible clindamycin resistance (MLSB i) was detected by the disk approximation (D test) test. The vancomycin MICs were determined by the E-test method with a 0.5 McFarland standard inoculum. The MIC clinical breakpoints were defined according to the CLSI guidelines (susceptible, ≤2 μg/ml; intermediate, 4–8 μg/ml; and resistant, ≥16 μg/ml). Results: Of the 72 S. aureus clinical isolates, 29 (40.2%) were MRSA. Inducible clindamycin resistance was detected in 16% of the MRSA isolates. Minimum inhibitory concentrations of vancomycin to the isolates of MRSA ranged from 0.125 μg/ml to 2 μg/ml. Conclusions: The rate of isolation of MRSA was high and it has emerged as a serious public health threat to Sri Lanka. The Minimum Inhibitory Concentration (MIC) of all the MRSA isolates were ≤2 μg/ml. None of the MRSA isolates were found to be intermediate-sensitive or vancomycin resistant. Therefore, vancomycin can be used as the drug of choice for treatment of infections caused by MRSA.