I 88 AGRICULTURAL BACTERIOLOGY. 



CHAPTER IX. 

 BACTERIA IN MILK. 



THE varieties of bacteria which get into milk from the sources 

 mentioned are very numerous. Some of them are of practically 

 universal occurrence in milk in wide localities. Others are quite 

 common, while still others are rare and must be looked upon 

 merely as occasional visitors. Only those which are some- 

 what common can be regarded as distinctive dairy bacteria. 

 It is impossible at present to give a list of even those bacteria 

 which are especially characteristic of dairy products. Some 

 two hundred or more different types have been described as 

 occurring in milk and its products, a considerable number of 

 which seem so common as to be properly regarded as dairy 

 species. Whether this large number can properly be regarded 

 as different species, or as different varieties, or as a few species 

 assuming different characters under different conditions, no one 

 would at the present day venture to assert. A knowledge of 

 these different forms is at the present time useless to any ex- 

 cept the technical bacteriologist. 



Of the long list of bacteria some are of scientific interest 

 only, while others are of very great practical importance. We 

 may leave out of consideration entirely most of the types 

 which are only rarely found in milk and whose relations to 

 dairy matters are slight. Those which are left, and whose re- 

 lations are important, may readily be divided into a few im- 

 portant classes according to their action upon milk. The 

 classes which are most important and easily recognized are as 

 follows : 



