In Vitro Activity of Mupirocin on Clinical Isolates of Staphylococcus aureus and its Potential Implications in Chronic Rhinosinusitis

Background: It has been postulated that bacterial biofilms are involved in the pathogenesis of chronic rhinosinusitis (CRS). Biofilms present on sinus mucosa are difficult to eradicate with conventional antibiotic therapy and are thought to provide a nidus for recurrent infection. Topical delivery of antibiotics via nasal irrigation may present a way of delivering high concentrations of anti-biofilm agents with potentially low systemic absorption and side effects. This study investigates the effectiveness of mupirocin and two other antibiotics, ciprofloxacin and vancomycin, on established in vitro biofilms of Staphylococcus aureus isolated from patients with CRS. Methods: S. aureus American Type Culture Collection 25923 and 12 clinical isolates were investigated for their ability to form biofilms in an in vitro setting using a 96 well microtiter crystal violet (CV) plate assay and confocal scanning laser microscopy (CSLM). Antimicrobial susceptibility tests to determine minimum inhibitory concentrations were performed on planktonic and biofilm forming strains. In addition, established biofilms were subjected to the antimicrobial agents at a twofold dilution series. A CV analysis of biofilm mass was performed after 1 and 24 hours of treatment, and minimum biofilm inhibition concentrations at 50% (MIB50) and 90% (MIB90) biofilm inhibition were recorded. Results: With use of a 96-well microtiter plate CV assay, 8 of the 12 clinical isolates formed mature biofilms after 8 days of culture. These results correlated with findings from CSLM analysis of in vitro biofilms grown on Permanox chamber slides. Increased antimicrobial resistance was observed in the biofilm isolates when compared with planktonic counterparts. Mupirocin was capable of reducing biofilm, mass by greater than 90% at concentrations of 125 mu g/mL or less in all S. aureus isolates. Ciprofloxacin and vancomycin were largely ineffective in attaining MIB90 concentrations within safe dosage ranges. Conclusions: The topical application of mupirocin via nasal irrigation may be useful in eliminating S. aureus biofilms present on the sinus mucosa of patients with CRS and may offer an additional treatment to patients with recalcitrant sinusitis.