Antimicrobial and antibiofilm activity of mupirocin and levomenthol against Staphylococcus aureus

Mupirocin is a topically and short-term used antibiotic, known for its antimicrobial activity against staphylococci, streptococci and some Gram-negative bacteria. Levomenthol is a plant derived cyclic alcohol which possesses antimicrobial activity. The aim was to study the antimicrobial activity of single mupirocin and levomenthol in vitro as well as their mixture towards planktonic bacteria and biofilm built by Staphylococcus aureus ATCC 29213 and five MRSA clinical isolates. Minimal inhibitory concentrations (MICs) of both compounds were determined using broth microdilution method according to CLSI guidelines with breakpoints determined using TTC reduction assay (1). Mupirocin was effective against all tested strains and MICs were between 0.125 and 0.250 μg/mL. MICs obtained for levomenthol was much higher, between 0.268 and 1.933 mg/mL. When both compounds were applied simultaneously, the antagonistic activity was recorded. However, when mupirocin was applied 1 hour after levomenthol treated S. aureus, MICs of both compounds for the majority of tested strains were lower. Biofilm inhibitory concentrations (BICs) were estimated by the MTT assay according to Walnecka et al (2). Both mupirocin and levomenthol applied alone showed antibiofilm activity in concentrations above 1 mg/mL, while their mixture exert stronger activity suggesting that synergistic or at least additive interactions might be implicated.