Comparative synergistic activity of cefoperazone, cefotaxime, moxalactam, and carbenicillin, combined with tobramycin, against Pseudomonas aeruginosa

A broth dilution checkerboard synergy assay was used to assess the activity of cefoperazone, cefotaxime, moxalactam, and carbenicillin, in combination with tobramycin, against 38 strains of Pseudomonas aeruginosa. Synergy occurred significantly more often (P less than 0.001) when tobramycin was combined with cefotaxime (63%) than when it was combined with carbenicillin (26%), cefoperazone (21%), or moxalactam (18%). Of the 25 synergistically inhibited strains, the combination cefotaxime-tobramycin was synergistic against 24 and was the only synergistic combination against 10. Of six strains initially resistant to cefotaxime, five were susceptible to this agent (minimum inhibitory concentration, less than or equal to 32 micrograms/ml) when it was combined with tobramycin. Clinical trials are needed to determine the therapeutic efficacy of cefotaxime-aminoglycoside combinations in the treatment of serious Pseudomonas infections.