204 



PHYSIOLOGY OF BACTERIA 



building material. The total solids of most bacteria 

 growing in the usual culture media are less than 0.03% of 

 the weight of the medium (see Table 132, p. 397). The 

 amount of construction material beyond this amount is 

 not likely to influence the rate of growth to any appreci- 

 able degree. With yeasts and molds, the crops, and 

 consequently the requirements, are larger. 



The energy available for growth is proportional to the 

 rate of fermentation, and it has been shown on pp. 114- 

 118 that beyond a certain minimal concentration of the 

 substrate, the rate of fermentation is almost independent 

 of the concentration. We must conclude from this that 

 the concentration of food is not likely to influence the 

 rate of growth very much, except when the concentration 

 becomes very low. 



The amount of available energy might be greatly 

 changed by using a different substrate, and examples 

 for this shall be shown in this chapter. But as long as 



