In vitro comparison of cefpirome and four other beta-lactam antibiotics alone and in combination with tobramycin against clinical isolates of Pseudomonas aeruginosa.

In vitro susceptibility studies of cefpirome versus cefotaxime, ceftazidime, imipenem, and piperacillin alone and in combination with tobramycin were performed against 153 clinical isolates of Pseudomonas aeruginosa from four medical centers. The minimal inhibitory concentration (MIC) for each antibiotic alone was determined by a standardized dilution method. Antibiotic combination studies were performed using a modified checkerboard technique. Cefpirome alone was more active (MIC90 64 micrograms/ml) than piperacillin (MIC90 128 micrograms/ml) or cefotaxime (MIC90 256 micrograms/ml) but less active than imipenem (MIC90 2 micrograms/ml) or ceftazidime (MIC90 32 micrograms/ml). The addition of tobramycin reduced the MICs of all of the beta-lactam antibiotics except for imipenem. The MIC90 for cefpirome when combined with tobramycin was 8 micrograms/ml compared to 16 micrograms/ml for cefotaxime and piperacillin, 8 micrograms/ml for ceftazidime, and 4 micrograms/ml for imipenem. The combination of tobramycin and cefpirome proved to be additive or synergistic for 82% of the isolates (highest rate) compared to 31% with imipenem (lowest rate). The potent in vitro antipseudomonal activity of cefpirome alone and in combination with an aminoglycoside (tobramycin) suggests that this agent may play a useful role in the therapy of infections due to P. aeruginosa.