Comparative activity of beta-lactam antibiotics against 'intrinsically' ticarcillin-resistant strains of Pseudomonas aeruginosa.

The in vitro activity of piperacillin, cefoperazone, cefsulodin, ceftazidime, aztreonam and imipenem was studied against 44 isolates of Pseudomonas aeruginosa with 'intrinsic' resistance to ticarcillin in comparison with 20 ticarcillin-susceptible strains, by MIC determination and the disk diffusion test. The activity of the antibiotics, imipenem excepted, against the resistant strains was reduced when compared to the susceptible strains. The most significant reduction was found for aztreonam and the least significant for ceftazidime. When considering the breakpoints, all strains were intermediate with aztreonam and cefoperazone, but most of them remained susceptible to ceftazidime, piperacillin and cefsulodin.