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 
