ILLINOIS STATE DAIRYMEN'S ASSOCIATION. 223 



rounded. For example, the lactic acid bacteria, which are the 

 most numerous about a dairy, and which cause milk to sour, 

 change the sugar of milk to lactic acid. After a certain amount 

 of acid or other product of growth has been developed, some 

 bacteria cannot longer thrive; the surroundings are so changed 

 by their own .operations that they cease to increase. This fact, 

 however, is not of much practical value to the milk producer; 

 the fermentation of his milk should never be allowed to proceed 

 so far that it stops itself. 



Bacteria cease to grow when in the presence of certain chem- 

 icals. When these are added to milk they are known as pre- 

 servatives; when they are used for such purposes as killing the 

 germs in or about a dairy they are known as disinfectants. Both 

 of these will be referred to later. 



DAIRY BACTERIA. 



The greatest number of bacteria are to be found where their 

 food is most abundant. Animals, feed, manure and milk are all 

 hosts or breeding grounds for bacteria. For this reason the 

 dairy is a place where myriads of germs of different kinds are to 

 be found. They must be always kept in mind, studied, and per- 

 sistently fought or controlled. 



NUMBER OF BACTERIA IN MILK. 



Milk ordinarily contains large numbers of bacteria. It is one 

 of the few media that is well adapted to almost any species and 

 quickly becomes inhabited with large numbers of those which 

 obtain entrance to it. There may be from a few hundred to 

 many million in a single drop, depending upon its exposure and 

 the time and opportunity the germs have had for increasing. 

 Dirt in milk is a sure sign of large numbers of bacteria. As the 

 rate of increase is influenced by temperature, the number pres- 

 ent at any time also depends much upon the previous tempera- 

 ture of the fluid. Russell has shown that the weather has a 



