Bacillus Acidi Lactici is concerned in the souring of milk. When milk is withdrawn from the mammary glands and exposed to the air, it is liable to have several forms of bacteria and fungi developing in it. An almost constant contamination is that with the lactic acid bacillus, which has the double effect of converting the milk sugar into lactic acid, and precipitating the casein, the latter effect being the result of the presence of the free acid. The result occurs in pure cultivations in sterilized milk. The bacillus is a small short plump rod, mostly joined in twos, seldom in longer chains. It is motionless. The bacilli form endospores. The bacillus grows on nutrient gelatine as a greyish white glittering layer, which does not liquefy the medium.

There are probably several other bacteria which convert sugar into lactic acid, but this is the chief agent.