Radiometric Estimation of the Replication Time of Bacteria in Culture: An Objective and Precise Approach to Quantitative Microbiology

In a recently developed, two-compartment liquid scintillation vial, the evolution of 14CO2 resulting from bacterial metabolism of uniformly labeled d-glucose was measured sensitively, cumulatively, and automatically in a liquid-scintillation counter. In each of eight species tested, a period of log-linear expansion of cumulative counting rate with time was observed. The exponential increase in cumulative counting rate was related to cell replication time by the integral of a first-order differential equation. Within a given species, the replication time measured by radioassay was found to be remarkably constant, unaffected by a fourfold variation in the activity of added labeled d-glucose, insensitive to the presence of carrier dextrose, and independent of the number of bacteria in the initial inoculum over a range of five orders of magnitude. These experiments demonstrate that the replication rate of an organism in culture is a highly reproducible characteristic that is susceptible to precise radiometric measurement in fundamental units of time under a variety of experimental conditions.