Activities of various beta-lactams and aminoglycosides, alone and in combination, against isolates of Pseudomonas aeruginosa from patients with cystic fibrosis.

The inhibitory and bactericidal activities of carbenicillin, ticarcillin, moxalactam, cefoperazone, azlocillin, piperacillin, ceftazidime, and three aminoglycosides, alone and in various combinations, were determined against 60 isolates of Pseudomonas aeruginosa from the sputum of patients with cystic fibrosis. Ceftazidime was the most active beta-lactam, with minimum inhibitory and bactericidal concentrations for 90% of isolates of 4 micrograms/ml. Moxalactam was the least active of the new beta-lactams, with activity equivalent to that of carbenicillin; each had a minimum inhibitory concentration for 90% of isolates of 64 micrograms/ml and a minimum bactericidal concentration for 90% of isolates of 128 microgram/ml. All combinations of an aminoglycoside plus a beta-lactam showed favorable inhibitory effects. Combinations of beta-lactams showed mostly addition or indifference. Although little antagonism was seen with combinations of beta-lactams or with aminoglycoside-beta-lactam combinations, no consistent advantage of beta-lactam combinations was demonstrated in vitro. These results suggest several single drugs and combinations that merit clinical evaluation in cystic fibrosis patients with Pseudomonas pulmonary infections.