lOO C. B. VAN NIEL 



It will be evident that even the growth of a microbe is subjected to 

 the "principle of limiting factors," If the synthesis of all cell con- 

 stituents can proceed faster in one medium than in another, growth will 

 obviously be faster in the former. Similarly, the rate at which a single 

 component is being synthesized may, under special circumstances, be 

 the determining factor for the rate of growth of the organism. In that 

 case our understanding of biosynthetic mechanisms might be advanced 

 by kinetic studies on growth, as well as by the more strictly chemical 

 ones which hitherto have been the sole method applied. And through a 

 kinetic approach it may be possible to learn something about the synthesis 

 of these cell constituents by organisms whose external environment does 

 not need to contain a supply of closely related substances because the 

 biosyntheses can proceed from a variety of simple ingredients of the 

 medium. 



While no investigations have yet been conducted with this purpose in 

 mind, some data are available which support the idea here advanced. For 

 example, Monod includes in his monograph (7) figures on the rate of 

 growth of Bacillus suhtilis in a complex and in a synthetic medium, the 

 latter composed of inorganic substances and sucrose. Growth proceeds 

 faster in the former than in the latter medium ; the generation times are 

 36 and 50 minutes respectively. Such differences could well be due to 

 differences in the rate at which particular cell constituents become 

 available, especially in view of the fact that in the synthetic solution they 

 must all be built up from inorganic ingredients and sucrose. If it were 

 possible to increase the growth rate in the simple medium through the 

 addition of known chemical entities, it might also become clear what 

 special syntheses are responsible for the lower growth rate. 



Another instance is provided by Whelton and Doudoroff's (39) 

 study on the assimilation of sugars and related compounds by Pseudo- 

 monas saccharophila. It includes a determination of the generation times 

 of this bacterium in a standard mineral medium with any one of a 

 number of carbon compounds. These were found to vary from 105-110 

 minutes in the presence of trehalose and sucrose, to 196 minutes in the 

 presence of acetate; cultures in glucose and lactate exhibited an inter- 

 mediate behavior with generation times of 178 and 136 minutes respec- 

 tively. Studies of this kind, if sufficiently extended, could yield informa- 

 tion concerning the nature of intermediate products which serve as 

 building blocks and the rate at which these are generated. Correlation of 

 such data with overall growth rates, supplemented with experiments on 

 the influence of special additions selected for the purpose of furnishing 



