Antimicrobial Activity of a Triple Antibiotic Combination Toward Ocular Pseudomonas aeruginosa Clinical Isolates

PURPOSE: Pseudomonas aeruginosa is a leading cause of corneal infections. Recently, we discovered an antimicrobial drug combination, polymyxin B/trimethoprim (PT) + rifampin, that displayed impressive efficacy toward P. aeruginosa in both in vitro and in vivo studies. As such, this combination was further evaluated as a potential keratitis therapeutic through testing the combination's efficacy against a diverse set of P. aeruginosa clinical isolates. METHODS: Minimum inhibitory concentrations (MICs) of moxifloxacin, levofloxacin, erythromycin, tobramycin, PT, polymyxin B (alone), trimethoprim (alone), and rifampin were determined for 154 ocular clinical P. aeruginosa isolates, 90% of which were derived from corneal scrapings. Additionally, the efficacy of PT + rifampin was evaluated utilizing fractional inhibitory concentration (FIC) testing. RESULTS: While 100% of isolates were resistant to erythromycin (average MIC 224 ± 110 µg·mL(−1)) and trimethoprim (alone) (206 ± 67.3 µg·mL(−1)), antibiotic resistance was generally found to be low: moxifloxacin (2% of isolates resistant; average MIC 1.08 ± 1.61 µg·mL(−1)), levofloxacin (3.9%; 1.02 ± 2.96 µg·mL(−1)), tobramycin (1%; 0.319 ± 1.31 µg·mL(−1)), polymyxin B (0%; 0.539 ± 0.206 µg·mL(−1)), PT (0%; 0.416 ± 0.135 µg·mL(−1)), and rifampin (0%; 23.4 ± 6.86 µg·mL(−1)). Additionally, FIC testing revealed that PT + rifampin eradicated 100% of isolates demonstrating additive or synergistic activity in 95% of isolates (average FIC index 0.701 ± 0.132). CONCLUSIONS: The drug combination of PT + rifampin was effective against a large panel of clinically relevant P. aeruginosa strains and, as such, may represent a promising therapeutic for P. aeruginosa keratitis. TRANSLATIONAL RELEVANCE: This work furthers the preclinical development of a novel antibiotic combination for the treatment of corneal infections (bacterial keratitis).