ILLINOIS STATE DAIRYMEN'S ASSOCIATION. 225 



in regard to the germs of manure, which are more abundant in 

 the stable than out of doors. As a result of their struggle for 

 existence, frequently a smaller number of species is in milk after 

 it has stood than when perfectly fresh, although the number of 

 individuals may have greatly increased. 



For practical purposes, dairy bacteria may be separated into 

 three classes, as follows: (i) Harmless bacteria; (2) useful 

 bacteria; (3) harmful bacteria. 



(i) Harmless bacteria, — These are the most numerous of the 

 forms found in milk. They are of comparitively small direct 

 importance to the milk producer, but they are not in milk when 

 first secreted and, as suggested above, if they obtain entrance 

 to it they are evidence that other germs also have had an oppor- 

 tunity to plant themselves. 



(2) Useful bacteria. — Some forms of bacteria are essential 

 to dairy operations. Cream is generally allowed to ripen or 

 sour before it is churned — in other words, useful bacteria are 

 given conditions favorable to their growth, and they cause acidl 

 to develop. Butter flavor depends upon several conditions, but. 

 one of the most important is the action of certain bacteria which^. 

 in the process of maturing or ripening, produce the desired! 

 aroma and flavor — cultures of bacteria for this purpose are now 

 regularly sold on the market. The chief differences between 

 varieties of cheese is caused by the kinds of bacteria that grow 

 in them. Bacteria needed in some cases are not wanted in 

 others, so the same species which are useful at one time may at. 

 another time be harmful. 



(3) Harmful bacteria. — These form the most important class.. 

 They may be subdivided into two groups, viz. , those having an! 

 injurious effect on the milk, and those not apparently affecting 

 the milk but having an injurious effect on the health of the con-- 

 sumer. Many species fall in only one of these subdivisions; 

 others belong to both. 



Certain bacteria may be indirectly injurious by producirg con-- 

 ditions favorable for other germs which are directly injurious,, 

 but not able to grow in milk until its nature has changed. For 



