Clinical significance of in vitro synergism between antibiotics in gram-negative infections.

Five combinations of antibiotics (ampicillin/gentamicin, cephalothin/gentamicin, carbenicillin/gentamicin, polymyxin/carbenicillin, and carbenicillin/cephalothin) were investigated in vitro and in 148 severe infectious episodes caused by gram-negative bacilli in patients with disseminated cancer. The use of combinations that were synergistic in vitro against the offending microorganism (synergy was defined as occurring when the minimal inhibitory concentration of each of the drugs in the combination was one-quarter or less of the minimal inhibitory concentration of each drug alone) was associated with a significantly better response to antibacterial therapy (P < 0.01) than the use of combinations that were not synergistic against the causative agent.