The study of the growth of bacterial cultures does not constiitute a specialized subject or branch of research: it is the basic method of Microbiology. It would be a foolish enterprise, and doomed to failure, to attempt reviewing briefly a "subject" which covers actually our whole discipline. Unless, of course, we considered the formal laws of growth for their own sake, an appproach which has repeatedly proved sterile. In the present review we shall consider bacterial growth as a method for the study of bacterial physiology and biochemistry. More precisely, we shall concern ourselves with the quantitative aspects of the method, with the interpretation of quantitative data referring to bacterial growth. Furthermore, we shall consider exclusively the positive phases of growth, since the study of bacterial "death," i.e., of the negative phases of growth, involves distinct problems and methhods. The discussion will be limited to populations considered genetically homogeneous. The problems of mutation and selection in· growing cultures have been excellently dealt with in recen t review articles by DelbrUck (1) and Luria (2).
No attempt is made at reviewing the literature on a subject which, as we have just seen, is not really a subject at all. The papers and results quoted have been selected as illustrations of the I points discussed.
DEFINITION OF GROWTH PHASES AND GROWTH CONSTANTS
DIVISION RATE AND GROWTH RATE
In all that follows, we shall define "cell concentration" as the number of individual cells per unit volume of a culture and ·'.'bacterial density" as the dry weight of cells per unit volume of a culture.
Consider a unit volume of a growing culture containing at time h a certain number Xl of cells. After a certain time has elapsed