Comparative in vitro activities of beta-lactam-tobramycin combinations against Pseudomonas aeruginosa and multidrug-resistant gram-negative enteric bacilli.

Piperacillin was more consistently active than tobramycin, carbenicillin, moxalactam, or ceftriaxone against strains of Pseudomonas aeruginosa isolated from blood cultures and against multidrug-resistant strains. Moxalactam and ceftriaxone were more consistently active than tobramycin, carbenicillin, or piperacillin against multidrug-resistant Enterobacteriaceae. Synergy between beta-lactam antibiotics and tobramycin was frequently observed against strains of P. aeruginosa isolated from blood cultures but not against multidrug-resistant organisms. Piperacillin plus tobramycin was the most active antibiotic combination against P. aeruginosa. Moxalactam plus tobramycin and ceftriaxone plus tobramycin were the most active antibiotic combinations against Enterobacteriaceae.