TYPES OF BACTERIA FOUND IN MILK T55 
TYPES OF BACTERIA FOUND IN MILK 
Many different types of bacteria get into milk from these vari- 
ous sources. Some of them are useful, some are of no particular 
significance, some are troublesome to the dairyman though not dis- 
tinctly harmful, while some are decidedly injurious either to the 
dairy products ortoman. A knowledge of these types is of primal 
importance to an understanding of their relations to dairying. 
The more important types are given in the following pages. For 
clearness and convenience we may divide them into three groups: 
(1) Normal milk bacteria. (2) Abnormal milk bacteria. (3) Dis- 
ease bacteria, The first two concern dairy problems only, while 
the last concerns the relation of milk to the public health. 
I. NORMAL MILK BACTERIA 
Under this head we refer to types of organisms that are practi- 
cally always present in milk and cannot be avoided by any ordinary 
means. They do not, of course, belong to the milk, but they are so 
widely distributed in barns and dairies that practically they can- 
not be avoided. There are very many different kinds among 
them, several scores at least having been described in milk from 
various localities. But they may be conveniently grouped and 
studied under three heads: 
Lactic Acid Bacteria—The most common fermentation of 
milk is its souring, a phenomenon so universal that it has been sup- 
posed to be a change belonging to milk itself. But it is now known 
to be produced always by the growth of bacteria. These organ- 
isms transform the milk sugar into lactic acid, a change that is 
sometimes expressed by the formula CeHO¢s = 2C3H,Os; but this 
(Sugar) (Lactic acid) 
equation fails to express the real nature of the change that occurs, 
which is much more complex. The fundamental phenomenon, 
however, is that the milk is made sour by the formation of lactic 
acid out of milk sugar. This is first seen in the appearance of a 
sour taste and later in the curdling. Milk contains its casein in a 
