62 VETERINARY BACTERIOLOGY 



The changes brought about by bacteria in nature are of such 

 importance that but for their continuance plant and animal 

 life on earth would quickly cease to exist. The fertility of the 

 soil and the consequent production of all food stuffs is directly 

 due to certain of the microorganisms present. 



Production of Alcohol. The various alcohols, but more par- 

 ticularly ethyl alcohol, are produced by certain bacteria, yeasts, 

 and molds. It has been shown that in the yeast the ability to 

 bring about this change is resident in the intracellular enzyme, 

 zymase. Probably similar enzymes are present in the alcohol- 

 producing bacteria and molds. The common bread or brewer's 



Fig. 27. Brewer's yeast, Saccharomyces cerevisice. 



yeast is the form most commonly used in the manufacture of 

 alcoholic beverages, but certain molds have been found very 

 useful in the production of alcohol for industrial purposes. Alco- 

 hol is commonly produced by the fermentation of one of the 

 hexose monosaccharids, such as dextrose. The reaction may be 

 given as follows: 



C 6 H 12 O 9 = 2C 2 H 5 OH + 2CO 2 . 



Dextrose. Alcohol. Carbon 



dioxid. 



Yeast is utilized for its other product of fermentation, carbon 

 dioxid, by the baker in bread making. Higher alcohols, such as 

 butylic and amylic, are also formed. The yeasts which produce 

 this change are quite widely distributed in nature, being par- 

 ticularly abundant on the surface of fruits and in saccharine 

 liquids. Fruit juices and other solutions containing sugar if 

 allowed to stand, therefore undergo " spontaneous " fermenta- 

 tion, with production of ciders, wines, and similar beverages. 



Production of Acids. Several of the organic acids are com- 

 monly produced by fermentative organisms. Three of these are 



