The radioactive thymidine incorporation method for the determination of antibiotic susceptibility of gram-negative bacilli.

The thymidine incorporation method was compared with the disc diffusion and broth dilution methods for determining the antibiotic susceptibility of Escherichia coli (233 strains), Proteus mirabilis (72 strains) and Klebsiella species (189 strains). The method measures changes in the rate of DNA synthesis of bacterial strains exposed to an antibiotic by comparing the amount of radioactive thymidine incorporated by these strains in relation to control strains. Good agreement was obtained between the results of the thymidine incorporation and disc diffusion methods for all antibiotics tested except streptomycin and tetracycline. Minimal inhibitory concentrations of antibiotics determined by the thymidine incorporation method after 3 and 5 hours were generally lower than those obtained after 24 hours by the conventional broth dilution procedure. The thymidine incorporation method may be potentially useful for determining the antibiotic susceptibility of bacteria a few hours after inoculation of a clinical specimen.