N-formimidoyl-thienamycin and norfloxacin against multiple-resistant Pseudomonas aeruginosa strains. Combined in vitro activity and comparison with 14 other antibiotics.

The in vitro susceptibility of 54 multiple-resistant strains of Pseudomonas aeruginosa to 16 antipseudomonal agents were determined. The majority of these organisms were also beta-lactamase-producing strains. Norfloxacin, N-formimidoyl-thienamycin and aztreonam showed the best antipseudomonal activity, with minimum inhibitory concentrations for 90% of isolates (MIC90) of 2, 8, and 8 mg/l respectively. Of the aminoglycosides, only amikacin (MIC90 = 32 mg/l) showed satisfactory activity, whereas among beta-lactam antibiotics, cefotaxime, ceftriaxone and moxalactam (MIC90 = 32 mg/l) were more active than cefoperazone (MIC90 = 128 mg/l), cefsulodin (MIC90 = 128 mg/l), piperacillin (MIC90 = 512 mg/l) and azlocillin (MIC90 = 1024 mg/l). A synergistic or partially synergistic interaction (fractional inhibitory concentration index less than 1) was demonstrated in vitro against 37% of the strains when N-formimidoyl-thienamycin was combined with norfloxacin.