MISCELLANEOUS LACTIC BACTERIA. 73 



lus. At moderate temperatures the souring is ordinarily 

 produced by this species. The second type is also almost 

 as widely distributed as the first, but it does not grow so 

 profusely in the milk, especially at moderate temperatures, 

 and is, therefore, not so universally the cause of milk sour- 

 ing. The other types are more or less unusual and play a 

 part in dairy processes which may be important, but is cer- 

 tainly secondary to the first two, although at higher temper- 

 atures some of the other species may be the significant types. 

 Each of the first two types is represented by a number of 

 varieties, but they are closely allied to each other and perhaps 

 should all be regarded as belonging to the same species. 



The amount of acid produced by different species of lactic 

 bacteria is variable, the same species appearing to differ 

 noticeably in this respect. The Bact. lactis acidi commonly 

 produces acid abundantly and causes a speedy souring and 

 curdling of the milk, but some varieties of this organism, 

 agreeing in all other respects, produce acid in smaller 

 amount and never in sufficient quantity to curdle the milk, 

 although it becomes sour to taste and .will curdle when 

 heated. Other species produce only a slight amount of 

 acid. The variations occur in different cultures of the same 

 species of bacterium, and may be produced by modifying 

 conditions. 



The power of producing acid seems to grow stronger as 

 the bacteria develop in the milk. If bacteria are taken from 

 fresh milk their power of forming acid is rather feeble, but 

 if bacteria are taken from the same milk after a two days' 

 growth, their power of producing acid is noticeably greater. 

 This seems to indicate that they get into the milk from some 

 source where they exist in a dormant condition, and only 

 develop their vigorous powers after growing in milk for 

 some hours. The conditions in the milk stimulate their 

 power of producing acid in marked degree. 



