Vancomycin creep and daptomycin minimum inhibitory concentration in methicillin-resistant staphylococcus aureus

Objective: To determine the in vitro activity of daptomycin and vancomycin against 50 strains of methicillin-resistant Staphylococcus aureus (MRSA) isolated from blood and pus specimens. Material and methods: Fifty consecutive MRSA were isolated from pus (n=25) and blood (n=25) were included in the study. Oxacillin susceptibility was determined by cefoxitin disc diffusion, green colored colonies on chromogenic media. Susceptibility testing for 18 antimicrobial agents was determined by a disc diffusion method. The minimum inhibitory concentration (MIC) of daptomycin and vancomycin was determined by the Etest as recommended by the Clinical and Laboratory Standards Institute (CLSI). Results: Antibiotic susceptibility pattern of the MRSA isolates showed that 38% were multi-drug resistant overall and 52% in blood and 24% in pus isolates when expressed separately. The MIC50 and MIC90 of daptomycin were 0.08 and 0.09 mg/L and of vancomycin were 1.2 mg/L and 1.3 mg/L, respectively. Ten percent of the isolates had vancomycin MIC of 2 mg/L which is the upper limit of CLSI breakpoint for sensitive isolates. None of the isolates showed intermediate susceptibility or resistance to vancomycin or daptomycin. Conclusion: Creeping MIC of vancomycin is a matter of concern and MIC of 1.5–2 mg/L of vancomycin in MRSA increases the risk of development of complicated bacteraemia. MIC's of vancomycin should be reported for all S. aureus isolates and should be used to guide treatment. Otherwise, daptomycin can be considered as an alternative antibiotic for therapy of MRSA infections in India.