WHAT ARI-; P.ACTERIA? 19 



fore, that of the three great food products upon which animals 

 feed, proteids, carbo-hydrates and fats, the bacteria feed espe- 

 cially upon the proteid foods, and but rarely upon the carbo- 

 hydrates. There is one conception of importance to the dairy- 

 man. Certain types of bacteria act upon milk sugar (a car- 

 bo-hydrate), converting it into lactic acid and causing the 

 milk to sour. There are some kinds that are capable of acting 

 on fats. 



The food which bacteria consume may be either living or 

 dead when they attack it. In the case of most bacteria the 

 organisms are unable to feed upon the material while it is alive. 

 If bacteria, for example, are placed upon living muscle, they are 

 unable to attack it and soon die; but if they are placed upon 

 the same muscle after it is dead, they feed upon it readily and 

 cause it to putrefy. These organisms which feed upon lifeless 

 bodies include the vast majority of bacteria. There are, how- 

 ever, other species of bacteria that are capable of living upon 

 the bodies of animals and plants while these are still alive. In- 

 asmuch as they can feed upon living organisms they are liable 

 to produce disease and constitute in general the disease bacteria. 

 Bacteria feeding upon living animals and plants are called 

 PARASITES. Bacteria feeding upon dead animals and plants are 

 called SAPROPHYTES. Both saprophytes and parasites are of 

 great importance in dairy problems. 



ENZYMES OR CHEMICAL FERMENTS 



In order to understand some of the topics discussed in the 

 following pages it is necessary to recognize the distinction be- 

 tween organized and unorganized ferments. The former term 

 applies to bacteria and yeasts which produce their action by 

 growth. They are actual living bodies which grow and multi- 

 ply. When they produce a fermentation, like the alcoholic or 

 the lactic acid fermentation, they increase in numbers during 

 the process ; that is, they act by 'growth. The unorganized fer- 



