2O PRACTICAL DAIRY BACTERIOLOGY 



ments, enzymes or chemical ferments, are not alive. They ap- 

 pear to be chemical substances which produce their fermenta- 

 tions by their simple presence. They do not live, grow or 

 multiply, but they have remarkable powers of bringing about 

 changes in other bodies. While these enzymes are not alive, 

 they are always produced by living bodies and from no other 

 source. For example, the living cells in the glands in the 

 stomach produce one of these enzymes, which we call pepsin, 

 and which has the power of greatly changing the chemical 

 nature of the swallowed food while it is in the stomach. Fre- 

 quently similar enzymes are secreted by bacteria. 



In the study of bacteria we are, of course, dealing primarily 

 with the living or organized ferments. But in some cases, as 

 we shall see, they produce their action directly through the 

 secreted enzymes, and thus in their study we have to consider 

 both classes of action. It is always important to bear in mind 

 that the organized ferments are ever increasing in numbers, 

 when they have proper food, while the enzymes do not thus 

 increase in amount, except as they are secreted by living cells. 

 If we stop the life of bacteria by an antiseptic, we stop its 

 fermentation, but such antiseptics will not stop the action of an 

 enzyme, since an enzyme acts chemically and not by its life. 



