212 PHYSIOLOGY OF BACTERIA 



sugar, an increase above 0.5% did not increase the growth 

 rate of Bad. typhosum. The increase from 1% to 2% 

 peptone produced with Bad. coli an increase in the growth 

 rate of about 3%, and even 4% peptone increased the 

 growth rate only 10-13%. Strept. ladis in diluted skim 

 milk grew at the same rate until the milk was diluted with 

 32 parts of water reducing the lactose concentration to 

 0.15% and the total protein to about 0.1%. Yeast in 

 beer wort showed almost the same rate of growth whether 

 the wort was normal or diluted with 12 volumes of 

 water. 



While the growth rate is changed very little, a slight 

 increase over a number of hours will produce noticeable 

 differences in the total number of cells in comparative 

 experiments. 



An important food for many bacteria is oxygen, and 

 on account of its low solubility, the oxygen concentra- 

 tion is frequently the limiting factor of the growth rate. 

 Organisms which cannot utilize oxygen, such as Strept. 

 ladis, are not benefitted in their growth by aeration 

 of the culture. 



SUMMARY OF THEORIES 



For theoretical considerations, we must distinguish 

 between the energy food and the building material though 

 in many cases, the two might be represented by the same 

 compound. 



The rate of fermentation had been found to be inde- 

 pendent of the substrate concentration above a com- 

 paratively low limit (with yeasts and streptococci, 

 about 1% glucose in the medium). If no increase in 

 available energy can be expected, an increase of the 

 growth rate through more concentrated food could come 

 only from a concentration of building material. Cell 



