The Estimation of Bacterial Populations with the Aid of a Photoelectric Densitometer

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Abstract
The construction and operation of a compensating photoelectric densitometer are described. Calibration of the instrument with serial dilutions of stock suspensions of microorganisms yields curves, when the density, D, is plotted against the concentration, C, which are very nearly straight lines. A simple relation is given by means of which corrections may be made for the slight deviations from linearity that are observed. The optical densities of suspensions of Lactobacillus acidophilus were detd. at intervals during the growth period. If the pH of the culture is held constant these measurements are proportional to the plate counts over an interval in which more than 90% of the total growth occurs.