Comparative in vitro synergistic activity of new beta-lactam antimicrobial agents and amikacin against Pseudomonas aeruginosa and Serratia marcescens.

The in vitro synergistic activities of moxalactam, cefoperazone, or cefotaxime in combination with amikacin or piperacillin were compared against aminoglycoside-susceptible and aminoglycoside-resistant isolates of Pseudomonas aeruginosa and Serratia marcescens by the checkerboard agar dilution method. All antimicrobial combinations demonstrated some synergy, and no antagonism was observed. Moxalactam plus amikacin and piperacillin plus amikacin were most frequently synergistic (two-thirds of the isolates inhibited synergistically by each combination), whereas combinations of moxalactam, cefotaxime, or cefoperazone with piperacillin were synergistic against only 18 to 25% of the isolates. Moxalactam plus amikacin was the combination most often synergistic for amikacin-susceptible P. aeruginosa, and piperacillin plus amikacin was the combination most frequently synergistic for amikacin-resistant P. aeruginosa and amikacin-susceptible S. marcescens. These results demonstrate frequent in vitro synergistic activity between the new beta-lactam agents and amikacin (especially moxalactam or piperacillin with amikacin), but comparative clinical trials are needed to establish the relative efficacy and toxicity of these combinations.