Effectiveness of topical antibiotics on Staphylococcus aureus biofilm in vitro

Background: In vitro biofilm-producing capacity in isolates of Staphylococcus aureus and Pseudomonas aeruginosa collected from the sinus cavities after endoscopic sinus surgery (ESS) are associated with a poor outcome in patients with chronic rhinosinusitis (CRS). However, conventional oral antibiotic therapy is frequently ineffective in eradicating bacteria in the biofilm form, Increasing the concentration of antibiotics may offer a means of countering this resistance. The aim of this study was to determine the in vitro activity of moxifloxacin (MOXI) against S. aureus in biofilm form (recovered from patients with CRS at least 1 year post-ESS), Method: This study was performed in a research microbiology laboratory, where five isolates of S. aureus with known biofilm-forming capacity were cultured in Tryptic Soy Broth 0.5% glucose in 96-well plates at 37 degrees C for 24 hours. After visual confirmation of biofilm formation, plates were incubated in phosphate-buffered saline (PBS) or with MOXI at concentrations of 0.1X, 1X, 100X, and 1000X minimal inhibitory concentration (MIC) for an additional 24 hours. Biofilm from 3 wells of each concentration were collected and sonicated and the number of viable bacteria was determined by serial dilution and plating. Results: After incubation, the number of viable bacteria was similar for nontreated and MOXI-treated biofilms at MIC and sub-MIC levels. However, MOXI at 1000X (0.1-0.2 mg/mL) gave a 2 to 2.5 log reduction in number of viable bacteria. Conclusion: In vitro results show that increased concentrations of antibiotics, easily attainable in topical solutions, are effective in killing bacteria in bacterial biofilms. This suggests a role for topical antibiotic therapies in the treatment of biofilm infections.