Antimicrobial effects of drugs against multidrug-resistant Pseudomonas aeruginosa

We evaluated the effects of antimicrobial drugs on four strains of Pseudomonas aeruginosa that are resistant to eight widely used antipseudomonal drugs (piperacillin, piperacillin-tazobactam, imipenem, meropenem, ceftazidime, aztreonam, amikacin, ciprofloxacin) and colistin. In the killing test, colistin (2 microg/ml) was the most effective, followed by aztreonam (48 microg/ml), piperacillin-tazobactam (192-4 microg/ml), piperacillin (192 microg/ml), and a three drug combination of azetreonam (16 microg/ml), ceftazidime (16 microg/ml), and amikacin (4 microg/ml). Six hours after drug addition, colistin (2 microg/ml), aztreonam (48 microg/ml), piperacillin-tazobactam (192-4 microg/ml), piperacillin (192 microg/ml), and the above three drug combination had bacteriostatic effects on all four strains. Colistin, three time breakpoint of aztreonam, piperacillin, or piperacillin-tazobactam, and the three drug combination of aztreonam, ceftazidime, and amikacin were effective in vitro.