Automatic radiometric measurement of antibiotic effect on bacterial growth

A new method for rapid, automatic radiometric measurement of antibiotic effects on bacterial growth was developed and compared with a conventional broth dilutior technique. The radiometric method measures the amount of radioactive CO(2) generated by the bacterial metabolism of (14)C-glucose in the presence of antibiotics. Antibiotic effect on bacterial growth was standardized by measuring the evolution of (14)CO(2) 3 hr after inoculation. This measurement was found to be quantitatively related to increasing concentration of antibiotic provided the organism was susceptible to the antibiotic tested. In 50 of 179 experiments (28%), each testing one organism against serial concentrations of an antibiotic, the concentration of antibiotic producing a 50% reduction of (14)CO(2) within 3 hr after inoculation in comparison with a control culture was the same as the minimal inhibitory concentration (MIC) determined by the broth dilution technique. In 129 experiments (72%), the antibiotic concentrations inhibiting (14)CO(2) release to 50% of the control level were less than the MIC values. Results of the radiometric method were related to those of the broth dilution method by constant factors characteristic of the organism and antibiotic tested. Our results indicate that the radiometric method provides a reproducible, quantitative, rapid, and sensitive measurement of the inhibitory effects of antibiotics on bacterial growth. The constant relationship between the results of the radiometric and conventional technique should facilitate the adaptation of the automated method to clinical testing of antibiotic susceptibility.