Technique for determining the bactericidal effect of drug combinations.

Two paper strips, each containing different antimicrobial agents, were placed on plates on Mueller-Hinton agar to permit antibiotic to enter the agar. A filter membrane was placed on this plate, and the microorganisms were planted on the membrane. After 6 h of incubation at 37 C, the membrane was transferred to antibiotic-free Mueller-Hinton agar containing triphenyltetrazolium hydrochloride and incubated for 18 h at 37 C. Specific growth patterns were indicative of additive (indifferent), synergistic, or antagonistic effects of the drug combination used. Trimethoprim and sulfamethoxazole proved to act synergistically against 85% of Escherichia coli, 86% of Klebsiella, and 89% of Proteus mirabilis strains tested. A few strains resistant to either drug were susceptible to their combination. The technique was useful against organisms with widely differing susceptibilities to the two antimicrobial agents tested.