Mupirocin at Subinhibitory Concentrations Induces Biofilm Formation in Staphylococcus aureus

OBJECTIVES: Mupirocin is a useful antibiotic against superficial skin infections. We compared the impact of mupirocin with a cephalosporin, a fluoroquinolone, an aminoglycoside, and a macrolide on planktonic cell growth and biofilm formation of methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA). MATERIALS AND METHODS: Minimum inhibitory concentration (MIC) of mupirocin was determined against S. aureus strains used in this study. Biofilm formation of S. aureus strains exposed to mupirocin was quantified by crystal violet staining assay. Moreover, biofilm structure and viability of the biofilm cells were visualized by Live/Dead staining assay. Biofilm-related gene expression was investigated by quantitative real-time PCR. RESULTS: MRSA USA300 clone was resistant to mupirocin with MIC of 1,024 mg/L, while MRSA ATCC-43300 and MSSA ATCC-29213 were susceptible with MICs of 0.03 mg/L. Planktonic cell growth of the S. aureus strains was inhibited by mupirocin in a dose-dependent manner. However, some of the low concentrations of mupirocin less than the MICs promoted biofilm formation. Confocal laser scanning microscopy of the biofilm structures and cell viabilities showed established biofilms of slightly higher cell density in the mupirocin treated groups, especially in the MRSA USA300 clone. Gene expression of RNAIII in planktonic cells and biofilms of MRSA USA300 clone showed the highest upregulation after initial exposure to sub-MIC of mupirocin followed by downregulation, whereas the other antibiotics showed various fluctuations. CONCLUSION: The results showed that subinhibitory concentrations of mupirocin promoted biofilm formation of S. aureus, in particular the MRSA USA300 clone.