Antimicrobial synergism in the therapy of gram-negative rod bacteremia.

To determine if antimicrobial synergism might affect the results of treatment of gram-negative rod infections, 444 bacteremias from 1972 through 1974 were studied. On these, 173 were treated with two antibiotics to which the infecting organisms were sensitive. Clinical responses were observed in 80% of 83 cases where antibiotic activity was synergistic, as defined by a minimum inhibitory concentration (MIC) of each antibiotic in combination being one-fourth or less than the MICs of individual drugs. This response rate was significantly better than the 64% response seen in patients treated with nonsynergistic combinations (p less than 0.05). Synergism correlated with significantly better clinical responses in those patients with "rapidly fatal" and "ultimately fatal" underlying disease (p less than 0.005), neutropenia (p less than 0.001), shock (p less than 0.01) and Pseudomonas aeruginosa infections (p less than 0.05). These results suggest that the use of antibiotic combinations to treat patients with gram-negative rod bacteremia who have the poorest prognosis is clinically justified and the improved results may be related to the synergistic activity of antimicrobial agents.