TYPES OF BACTERIA FOUND IN MILK. 143 
cance, some are troublesome to the dairyman though not distinctly 
harmful, while some are decidedly injurious either to the dairy prod- 
ucts or to man. A knowledge of these types is of primal impor- 
tance 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: i. Normal 
milk bacteria. 2. Abnormal milk bacteria. 3. Disease 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 practically 
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 cannot 
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 organisms 
transform the milk sugar into lactic acid, a change that is sometimes 
expressed by the formula C 6 H I2 O 6 = 2C 3 H 6 O 3 ; but this equation 
(Sugar) (Lactic acid) 
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 state of partial solu- 
tion, but if the milk is made sufficiently acid the casein can no longer 
remain in solution and is precipitated. The precipitation of casein 
is the curdling of milk, and it occurs when we add to it any kind of 
acid. In the normal souring of milk, when the acid reaches 0.7 per 
cent, to 0.9 per cent, the milk curdles. 
