Comparative susceptibilities of Pseudomonas aeruginosa to 1-oxacephalosporin (LY 127935) and eight other antipseudomonal antimicrobial agents (old and new)

In vitro susceptibilities of 53 clinical isolates of Pseudomonas aeruginosa to nine antipseudomonal antibiotics were determined. From 96 to 100% of the isolates were susceptible to piperacillin, ticarcillin, and 1-oxacephalosporin (LY 127935). Of the aminoglycosides, 89, 82, 79, and 29% were susceptible to amikacin, tobramycin, gentamicin, and netilmicin, respectively. A total of 96% and 78% of the isolates were susceptible to 1-oxacephalosporin (LY127935) and cefotaxime, respectively, at concentrations of 62.5 micrograms/l. Supplementation of testing media by calcium and magnesium not only markedly increased the minimal inhibitory concentrations of the aminoglycosides, but also raised those of cefotaxime and the penicillins; no significant effect was noted with 1-oxace-phalosporin. Synergy was not demonstrated consistently in vitro with 1-oxace-phalosporin combined with either carbenicillin, ticarcillin, gentamicin, or tobramycin.