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 J2 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. 



