Norfloxacin and ursolic acid: in vitro association and postantibiotic effect against Staphylococcus aureus.

Aims: We investigated the effectiveness in vitro of the association between norfloxacin (NOR) and ursolic acid (UA) against Staphylococcus aureus. Methods and Results: The minimal inhibitory concentrations (MICs), the minimal bactericidal concentrations, the bacterial killing and the postantibiotic effect (PAE) of NOR and UA were determined both singly and in combination. A synergistic interaction was observed against Staph. aureus ATCC 29213: the mean PAEs were 3h for NOR, -1·2h for UA (1×MIC) and 2·0h for UA (2×MIC). Synergism was observed with longer PAEs and postantibiotic sub-MIC effects after NOR/UA exposure. UA was also active against clinical isolates and methicillin-resistant Staph. aureus. Conclusions: The application of antimicrobial combinations may address the rising resistance to established classes of both systemic and topical agents. Significance and Impact of the Study: In vitro interactions between NOR and UA may contribute to the development of novel topical agents for the treatment of skin infections as well as for topical formulations.