Treatment of Staphylococcus Aureus with Antibiotic and Quorum-Sensing Inhibitor Combinations Reduces Severity of Keratitis

Purpose: To examine whether treatment with ciprofloxacin and fimbrolides improves the outcome of Staphylococcus aureus keratitis. Methods: A S. aureus strain was tested for the minimum inhibitory concentration (MIC) of ciprofloxacin and fimbrolides, and whether they had synergistic antimicrobial activity. The effect of fimbrolides on production of α-toxin was measured after growth at sub-MIC and testing supernatants for hemolysis. Cytotoxicity of the fimbrolides was measured by adding increasing concentrations to corneal epithelial cells and measuring release of lactate dehydrogenase. Ten hours after scratching corneas of mice and applying bacteria, treatment was initiated with vehicle, ciprofloxacin, fimbrolide, or a combination of ciprofloxacin and fimbrolide. The number of infecting S. aureus cells was measured after 6 hours treatment, and clinical responses were recorded (maximum score 20). Results: The MIC of the fimbrolides was 6.25-12.5 μM and growth in fimbrolides reduced production of α-toxin by > 90%. Fimbrolides were only cytotoxic at > 25 μM. Treatment of keratitis with ciprofloxacin significantly (p < 0.05) reduced numbers of S. aureus in corneas. Although there was minimal synergism between the two antimicrobials in vitro and treatment with the fimbrolide/ciprofloxacin combination in vivo did not significantly reduce numbers of S. aureus, there was a significantly reduced clinical score (by > 45%; p < 0.05) of the combination compared to single application of antimicrobials or vehicle control (average 4.25 ± 0.35). Conclusion: Application of non-toxic concentrations of quorum-sensing inhibitor fimbrolides in combination with ciprofloxacin reduce the numbers infecting bacteria to the same extent as ciprofloxacin alone, but also significantly improved the clinical outcome of S. aureus keratitis. Fimbrolides may be useful adjunct agents to treat keratitis.