The Three Groups of Bacteria 371 



may find their way into the milk from various sources. 

 The soil harbors not a few of them, and others are found 

 in manure, in decaying vegetable and animal substances, 

 in the water of streams and wells, and on the roots and 

 stems of plants. They frequently gain access to the milk 

 and multiply in it to a greater or slighter extent. Yet, 

 under normal conditions, they seldom gain the upper 

 hand and are forced by the lactic-acid bacteria to assume 

 a secondary place. 



The third group is composed of bacteria that cause 

 neither the acid nor sweet curdling of milk, nor do they 

 produce casein-digesting enzymes. Under certain con- 

 ditions, they may apparently multiply in the milk to 

 very considerable numbers without producing any 

 marked change in its taste or appearance. Occasion- 

 ally, some of these species find lodgment in the cow's 

 udder, and occur thus in large numbers in freshly drawn 

 milk. However, like the members of the preceding 

 group, they are rapidly crowded back by the lactic- 

 acid bacteria and do not play a prominent part in the 

 subsequent changes which the milk undergoes. 



Lactic-acid bacteria. — The composition of the milk is 

 pecuHarly favorable for the rapid growth of the lactic- 

 acid bacteria. No matter what the initial contamina- 

 tion may be, the latter are almost always certain to 

 make their appearance and to cause those changes that 

 lead to the suppression of the organisms of the second 

 and third groups, as well as of others not included in 

 them. 



The powerful weapon possessed by the lactic-acid 

 bacteria for the crowding back of other species consists 



