96 C.B.VANNIEL 



develop in a strictly defined medium, it was possible to determine the 

 effect of variations in the concentration of a single nutrient constituent 

 upon both the rate of growth and the final crop. So far, this approach 

 has been restricted to experiments with sugars and related compounds, 

 applied over a wide range of concentrations. With three different bac- 

 terial species it was established that, for a given carbon source, the rate 

 of growth is independent of the substrate concentration, and the total 

 yield is strictly proportional to it, down to such low levels as 20 mg. 

 substrate per liter. 



There are on record a number of observations which suggest a dif- 

 ferent response of microorganisms to the concentration of nutrients. 

 Thorne (14), for example, claims to have shown that the rate of yeast 

 growth in sugar media falls uniformly with increasing concentration of 

 the yeast, a behavior entirely at variance with the above-mentioned 

 results. And Porter (15) has recently resuscitated the concept of "bio- 

 logical space" as earlier developed by Bail (16). The argument here is 

 that, because the total crop of bacteria in liquid cultures reaches a cer- 

 tain maximum regardless of the concentration of foodstuffs, growth 

 must be limited by a space factor rather than by nutrients. The follow- 

 ing passage from Porter's book (15, p. 136) is worth quoting in this 

 connection : 



"There is considerable evidence in favor of some of Bail's claims 

 concerning space theory and Af (maximum) -concentration, but many 

 of his points are not accepted at this time. Further work will have to be 

 done on this subject — why bacteria stop dividing at a maximum speed 

 when a certain population is reached — before any definite conclusions 

 can be reached." 



Most, if not all, of these aberrant results can be readily ascribed to the 

 experimental conditions imposing a limiting factor which, in spite of 

 repeated exhortations, is still insufficiently taken into account. This 

 limiting factor is oxygen. The supply of this gas is governed by its 

 very low solubility coefficient and unless special measures are adopted 

 to insure a greatly increased rate of diffusion (e.g. by forced aeration, 

 continuous shaking, etc.), its availability in liquid media is apt to fall 

 below that required for an optimum growth rate of the organisms. The 

 studies of Phelps (11), Rottier (17), Rahn and Richardson (18), and 

 Anderson (19) attest to the correctness of this statement. It seems more 

 than likely that the "biological space" referred to above is simply a 

 measure of the oxygen supply. Particularly the close packing of bacteria 

 in colonies provides a convincing argument for the contention that 



