Comparison of bactericidal activity of selected β-lactam antimicrobials against Pseudomonas aeruginosa

The efficacy of ceftazidime, piperacillin, tobramycin and ticarcillin was tested in vitro against 102 clinical isolates of Pseudomonas aeruginosa. The MIC and MBC of each agent alone were determined. In addition, the rate of killing of 27 isolates by each beta-lactam agent was tested both singly and in combination with tobramycin. The concentrations of beta-lactams and of tobramycin were tailored to each organism; these were double the MBC in the case of the beta-lactams and half the MBC for tobramycin. Although 85.2% of isolates were susceptible to piperacillin and only 58.8% were susceptible to ticarcillin, the MBC90 was higher for piperacillin (greater than 256 mg/l) than ticarcillin (199.7 mg/l). The addition of tobramycin in sublethal concentrations increased the rapidity of killing of bacteria.