IN VITRO ACTIVITY AND EMERGENCE OF RESISTANCE TO MUPIROCIN IN STAPHYLOCOCCUS AUREUS

Background. Nasal carriage of Staphylococcus aureus is an important risk factor for later infection with the same strain. Topical mupirocin is used for S. aureus nasal decolonization. However, due to increasing mupirocin misuse resistance may develop rapidly. We determined the in vitro activity of mupirocin and compared the emergence of resistance among 30 methicillin-susceptible S. aureus (MSSA) isolates and 30 methicillin-resistant S. aureus (MRSA) isolates. Methods. Minimal inhibitory concentrations (MICs) were determined according to the National Committee for Clinical Laboratory Standards (NCCLS). Emergence of resistance studies were performed by incubating the isolates with increasing concentrations of mupirocin (0.125 to 16 µg/ml) over 8 days in Mueller-Hinton broth (MHB). Isolates were screened for resistance using Mueller-Hinton agar plates with 4 µg/ml of mupirocin. Results. MICs were determined for all recovered isolates. Before mupirocin exposure, MSSA and MRSA MIC90 (range) in µg/ml were 0.25 (0.06–4) and 0.25 (0.06–0.5). After one day of mupirocin exposure, all recovered isolates demonstrated decreased susceptibility to mupirocin (MSSA MIC90 64 µg/ml and MRSA MIC90 32 µg/ml). On subsequent days, no further significant changes in the mupirocin MIC90 was detected. Conclusions. The in vitro study suggests that mupirocin resistance emerges easily and early after exposure to low antimicrobial concentrations. Therefore, mupirocin should be used prudently and should always be combined with other decolonization interventions such as gargling and skin wash with chlohexidine.