Combination effect of levofloxacin and cefmenoxime against ocular isolates of Pseudomonas aeruginosa.

Purpose: This study aimed to investigate the effects of the combination of levofloxacin (LVFX) and cefmenoxime (CMX) on ocular isolates of Pseudomonas aeruginosa.

Methods: Twenty-three strains of P. aeruginosa isolated from microbial keratitis cases in Australia were utilized in this study. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of each antibiotic alone were determined and the combined susceptibility of antibiotics was assessed through fractional inhibitory concentration index (FICI). Additionally, tolerance of a ciprofloxacin tolerant P. aeruginosa isolate to each antibiotic alone and combination were analyzed.

Results: The median MIC and MBC values of CMX against P. aeruginosa were 16 µg/mL and 32 µg/mL, respectively. The median MIC and MBC values of LVFX were 0.5 µg/mL and 1 µg/mL, respectively, and all isolates were susceptible. In dual combination, the median of FICI of all strains was 0.63; seven strains showed synergistic effects (FICI≤0.5), and fifteen strains showed additive activities, while no strains exhibited antagonism. The isolate of P. aeruginosa that had previously exhibited tolerance to ciprofloxacin showed high tolerance to CMX alone and low tolerance to LVFX alone. This strain showed low tolerance to the combination of CMX and LVFX.

Conclusion: The combination of LVFX with CMX may have beneficial interactions which increase antibacterial activity against P. aeruginosa isolates and may be a feasible option for addressing the issue of bacteria resistant to each antibiotic.