18 Dairy Bacteriology. 



break down such enormous quantities of organic mat- 

 ter. Most of these enzymes react toward heat, cold, and 

 chemical poisons in a manner quite similar to the living 

 cells. In one respect, they are readily differentiated, and 

 that is, that practically all of them are capable of pro- 

 ducing their characteristic chemical transformations un- 

 der conditions where the activity of the cell is wholly 

 suspended as in a saturated ether or chloroform atmos- 

 phere. The production of enzymes is not confined to 

 bacteria, but they are found throughout the animal and 

 plant world, especially in those processes that are con- 

 cerned in digestion. Rennet, used in cheese making, is 

 an example of an animal enzyme. 



Distribution of bacteria. As bacteria possess greater 

 powers of resistance than almost any other form of life, 

 they are found very widely distributed over the surface 

 of the earth. In soil they are abundant, because of the 

 fact that all of the conditions necessary for growth are 

 here best satisfied. They are, however, distributed with 

 reference to the layers of the soil; the soil proper, i. e., 

 that turned over by the plow, is extremely rich in them 

 on account of the abundance of organic matter. But at 

 the depth of a few feet they decrease rapidly in num- 

 bers, and in the deeper layers, from six to ten feet, or 

 more, they are normally not present, because of the lack 

 of x proper food supply and oxygen. The fertility of 

 the soil is closely associated with their presence. 



The bacteria are found in the .air because of their de- 

 velopment in the soil below. They are unable to grow 

 even in a moist atmosphere, but are so readily dislodged 

 by wind currents from the soil that over land areas the 

 lower strata of the air always contain them. They are 

 more numerous in summer than in winter; city air con- 



