206 PHYSIOLOGY OF BACTERIA 



What happens in a very dilute medium, can hardly 

 be explained theoretically, as we know but very little 

 about the rates of fermentation at very low concentra- 

 tions of fermentable material. We may assume in the 

 simplest case, that it is proportional to the amount of 

 material ; the available energy would then be proportional 

 to the peptone concentration. This may account for 

 the fairly close agreement between rate of growth and 

 concentration in Table 50, causing the product of 

 generation time and food concentration to be nearly 

 constant. It seems that even in these low concentra- 

 tions, there is still sufficient building material to allow 

 cell growth, if only the energy suffices for the needs of 

 the cell. But we may also assume that the cells use the 

 energy much more economically if the food supply is 

 scanty. The question can be settled only by experiments 

 where the energy supply is constant, i.e., where the 

 concentration of fermentable material is fairly high, 

 while the building material is varied. Such experiments 

 are still lacking. 



Other examples can be found in the experiments by Curran (1925) 

 who cultivated Bad. coli and Bad. aerogenes in solutions of 1, 2 and 

 4% peptone. An increase of 100% in the food concentration caused 

 an increase of the growth rate of only 3 %, and an increase of 300 % 

 in the food increased the growth rate 13%. 



A similar ratio was obtained with yeast by Zikes (1919a). He 

 studied the growth of beer yeast in different concentrations of wort 

 by direct observation under the microscope, and obtained the follow- 

 ing relative growth rates: 



for 1 part of wort with parts water: relative growthrate 100 

 1 part of wort with 4 parts water: relative growthrate 102 

 1 part of wort with 8 parts water: relative growthrate 94 

 1 part of wort with 12 parts water: relative growthrate 98, 95, 



and 92 



