KINETICS OF GROWTH Id 



similar or different building blocks, might then reveal the nature of 

 limiting synthetic reactions. 



I do not mean to convey the impression that the above-suggested 

 approach is an easy and certain way of solving the problem of growth. 

 They are intended to emphasize that kinetic studies of the growth of 

 microorganisms, hitherto rather neglected, should be seriously con- 

 sidered as potentially capable of supplying information which can con- 

 tribute toward a moie detailed analysis of the manifold and interlinking 

 events which, in their totality, constitute the growth process. 



The foregoing reflections on the effects of external factors on the 

 growth rate of microorganisms, and on the interpretations and deduc- 

 tions that can be derived from the observed results, may have fostered 

 the impression that internal factors can be regarded as more or less 

 constant. But this is not the case; there exists an impressive body of 

 indisputable evidence to the contrary. Changes in the morphological, 

 physiological, and biochemical behavior of microbial cultures, attributed 

 to adaptations, modifications, mutations, etc., have long been recognized 

 as manifestations of the intrinsic variability of the organisms. Monod's 

 recent studies (7, 40-42) on the growth rate of bacteria in media con- 

 taining two different carbon sources instead of a single one offer an 

 important advance in this field. 



Depending upon the nature of the organism and of the substrates, 

 either of two types of growth curves describes the observed response 

 of the bacteria. One represents a single period of exponential growth 

 extending over the entire time interval during which substrate is avail- 

 able, with the sharp break at the end marking the complete utilization of 

 the two carbon sources. The other kind consists of two distinct parts, 

 each in itself a characteristic growth curve, with its phases of acceler- 

 ated, maximal, and retarded growth rates, the two curves being 

 conspicuously separated by a horizontal stretch which indicates the 

 cessation of growth during the corresponding period. Monod has desig- 

 nated this latter behavior by the term "phenomene de diauxie" ("di- 

 phasic growth") and has shown that it involves the occurrence of an 

 adaptation process. 



By chemical analyses of the medium at different times it was ascer- 

 tained that the first part of the growth curve corresponds to growth of 

 the organism at the expense of only one of the two substrates, to the 

 complete exclusion of the other. The second, separate growth curve 

 coincides with the decomposition of the second substrate. And the inter- 

 vening period during which the population remains constant represents 



