BACTERIA OF MILK 57 



lower temperatures down to 20 C. (68 F.). They do 

 not grow as well as the Streptococcus lacticus between 

 15 and 20 C. (59 and 68 F.), and at lower tempera- 

 tures the difference is still greater. 



These organisms are normal inhabitants of the in- 

 testines of the cow and consequently are hardly ever en- 

 tirely absent from milk. They are present in water pol- 

 luted by drainage from barnyards, manure heaps and 

 cesspools, and also on field crops, especially roots grown 

 on manured ground. Their presence in milk in any 

 considerable number indicates that it has been contami- 

 nated with manure or with polluted water. 



Milk also contains anaerobic bacteria which ferment 

 lactose and its salts, forming gas in large quantity and 

 producing strong-smelling acids like butyric, valerianic 

 and propionic. These organisms are present ordinarily 

 in small number and their development is usually pre- 

 vented by the acid-forming bacteria. When they grow 

 in milk in large numbers, a curd containing many gas 

 bubbles is formed. The milk has the odor of the acid 

 produced and frequently an odor of putrefaction also. 

 Because of the latter condition, these organisms are re- 

 garded as putrefactive bacteria. The best known are 

 those which produce butyric acid and are consequently 

 called butyric acid bacteria. They are very large spore- 

 forming bacilli which live in cultivated soil in symbiosis 

 with the peptonizing bacteria. They are usually present 

 in the spore-forming stage on the products of the field. 

 Morphologically, they are distinguished from the other 

 spore-forming milk bacteria by a change in form during 

 spore formation, becoming shuttle-shape, drum-stick- 

 shape, etc. 



Ayers and Johnson found gas-forming bacteria in 



