I02 . C. B. VAN NIEL 



the interval needed by the bacteria to adapt themselves to the utilization 

 of this second substance. 



Adaptations of this sort have been known for several years and have 

 been ascribed to an adaptive enzyme formation (43, 44). They are 

 strictly dependent upon the presence of a specific substrate, an observa- 

 tion which has led Yudkin (45) to develop an ingenious hypothesis 

 concerning the mechanism by which adaptive enzymes are generated, 

 based upon differences in stability of the free enzyme molecules and of 

 the enzyme-substrate complex. Monod's experiments (40) have shown 

 that the rate of such adaptations in bacterial cultures depends not only 

 upon the presence but also upon the concentration of the substrate. In 

 view of the well-established fact that metabolic rates are generally in- 

 dependent of this last-named factor, and also that the growth rate of 

 the adapted bacteria is constant over a wide concentration range, 

 Yudkin's hypothesis may have to be modified. 



Later experiments by Monod (42) have made it probable that adap- 

 tive enzyme formation must be interpreted as a competitive phenomenon. 

 This follows from observations on diphasic growth in relation to 

 variations in the concentration ratio of the two substrates. The higher 

 this ratio in favor of the substrate inducing the adaptation, the shorter 

 is the period of suspended growth : the measure of the adaptation rate. 

 The lag can even be eliminated completely by sufficiently magnifying the 

 difference in concentrations of the substrates. 



From these results it has been inferred that the two different sub- 

 strates compete for a common precursor of the specific enzymes. Dif- 

 ferences in affinity of the substrates for the precursor would then be 

 responsible for the quantitative variations in the enzyme systems of 

 organisms exposed to solutions which contain both substrates simul- 

 taneously but in different concentration ratios. 



Growth is the expression par excellence of the dynamic nature of 

 living organisms. Among the general methods available for the scientific 

 investigation of dynamic phenomena, the most useful ones are those 

 which deal with the kinetic aspects. The relative paucity of such data 

 on the growth of microorganisms at the present time cannot be denied. 



Nevertheless a few contributions of considerable importance have 

 been made in this respect. It can also be asserted with some confidence 

 that the recent advances in methodology for the determination of growth 

 curves will help in overcoming the justifiable hesitation on the part of 

 investigators, previously confronted with cumbersome techniques, to 

 adopt this approach for future studies on microbial growth. Only by 



