Lactic-Acid Bacteria 373 



of the bacteria, the rate of souring is also affected by 

 the temperature at which the milk is kept. 



These facts show, then, that different lactic-acid 

 organisms may be prominent at different temperatures. 

 It may be likewise noted in this connection that the sup- 

 ply of air also helps to determine the predominance of 

 one or another of the lactic-acid species. For example, 

 B. lactis acidi will grow where air is freely supplied, 

 whereas Bacillus casei E will grow best where the air 

 is excluded. 



Notwithstanding the crowding out of the non-acid 

 species by the lactic-acid bacteria in milk undergoing 

 a normal transformation, the former are of some im- 

 portance in influencing the rate of souring. It has been 

 shown by Marshall that certain of these organisms, 

 though themselves incapable of producing acid, encour- 

 age, to a very marked degree, its formation by the lactic- 

 acid bacteria. Moreover, when such non-acid bacteria 

 are allowed to grow in the milk for some time, and the 

 latter is then sterilized and again inoculated with a pure 

 culture of a lactic-acid bacterium, the formation of lactic 

 acid will proceed more rapidly than in a similar portion 

 of milk in which the non-acid bacteria have not been 

 allowed to grow previously. 



The spontaneous souring of milk is not, therefore, 

 as simple a process as is commonly supposed. Promi- 

 nent as are the lactic-acid bacteria in such transforma- 

 tions, still they are encouraged in their development by 

 the numerically unimportant non-acid species that 

 probably prove serviceable in spHtting the complex 

 nitrogenous substances of the milk. It is likely that the 



