[In vitro comparative activity of 4 beta-lactams against Pseudomonas aeruginosa. Role of cefoperazone].

The authors compared in vitro activity of cefoperazone, piperacillin, carbenicillin and ceftazidime against 100 clinical isolates of Pseudomonas aeruginosa. Minimal inhibitory concentrations (MICs) were determined by the agar dilution method. Minimal bactericidal concentrations were measured using microplates. Eighty-two percent of strains were susceptible or intermediate to cefoperazone, with MICs less than 30 micrograms/ml. The geometric mean of the MICs of cefoperazone was 14.3 micrograms/ml; MIC 50 was 6.9 micrograms/ml. Activity of cefoperazone was comparable to that of carbenicillin, to which eighty-three percent of strains were susceptible, with MICs less than or equal to 128 micrograms/ml. Ceftazidime was also highly active against Pseudomonas, with a mean MIC of 3.29 micrograms/ml. MIC 50 was 1.9 micrograms/ml. Piperacillin was slightly less active, with MICs less than or equal to 32 micrograms/ml for 35% of strains. Cefoperazone exhibited bactericidal activity on 50% of strains at a 28.8 micrograms/ml concentration. These values must be interpreted with caution as they do not allow for the phenomenon of regrowth of Pseudomonas aeruginosa due to the presence of microaggregates of the bacteria in vivo despite bactericidal concentrations of antibiotic.