208 PHYSIOLOGY OF BACTERIA 



The fermenting capacity of the cells fluctuates considerably, 

 but does not decrease distinctly until the milk is diluted 1:8; i.e., 

 the rate of fermentation is constant until the sugar concentration 

 falls below 1%. If we average the generation times (omitting the 

 lag period from 0-6 hours), we find the growth rate also quite uniform, 

 decreasing very slowly until the milk is diluted with 32 parts of water. 

 At this point, the lactose concentration is 0.15%, and the total 

 proteins amount to about 0.1%, of which the available proteins are 

 only a very small fraction. 



A frequently disregarded factor is the oxygen concen- 

 tration. Many bacteria grow in peptone solution only 

 aerobically, i.e. they can obtain energy from peptone 

 only by oxidizing it. A medium containing no carbohy- 

 drates or other hydrogen acceptors can support growth 

 of facultative organisms only as long as oxygen is avail- 

 able. As soon as sufficient growth has developed to make 

 the culture distinctly cloudy, all oxygen will have been 

 used up in the medium, and growth can take place only 

 at the very surface (see p. 80). 



This is illustrated by an experiment of Rahn (1912) 

 who inoculated a 1 % peptone solution with an unidenti- 

 fied aerobic soil bacterium. Part of this inoculated 

 solution was kept in a flask, and part was poured on 

 sterile sand so that the sand contained 10% moisture; 

 this made the moisture films around the sand particles 

 in which the bacteria grew about 20 to 40m thick, and 

 gave optimal conditions for a rapid oxygen exchange. 

 Table 52 shows one of the three experiments. 



The same experiment was repeated with Strept. 

 lactis in milk. This organism does not have the faculty 

 of utilizing oxygen, and the better oxygen supply did 

 not increase growth nor acid formation. On the con- 

 trary, a slight retardation through the better oxygen 

 exchange became noticeable. 



