Pseudomonas aeruginosa isolated from corneal ulcer: susceptibility to antimicrobial agents tested alone or in combination.

Twenty-eight isolates of P. aeruginosa from corneal ulcers were tested for their susceptibility to 19 antimicrobial agents by an agar dilution method. The effect of a combination of an aminoglycoside with a beta-lactam, ciprofloxacin or colistin was checked by the checkerboard titration method. The minimal inhibitory concentrations (MICs) of these antimicrobial agents varied greatly. The MICs of aminoglycosides were near the breakpoint value of the usual susceptibility definition in most tested strains, but some were highly resistant. Ciprofloxacin, imipenem, aztreonam, ceftazidime, cefoperazone, piperacillin and colistin were other effective drugs with low MICs. Piperacillin, ceftazidime or aztreonam in combination with gentamicin or amikacin revealed a synergistic effect, but antagonism was found with ciprofloxacin or colistin combined with gentamicin or amikacin. Considering the difference in drug concentration achievable between systemic administration and topical application, the MIC quantitative determinations and the combination effect of antimicrobial agents against ocular isolates could provide useful information to ophthalmologists in choosing drugs for treatment of P. aeruginosa ocular infections.