PHYSIOLOGY OF MICRO-ORGANISMS 169 



organisms are due to chemical changes produced by them. 

 Primary products are those which are produced inside the cell 

 by its living protoplasm. These include all the synthetic products 

 such as the substance of the germ itself, the complex bodies 

 which it forms from simpler substances, such as its enzymes 

 and its toxins, and also the simpler chemical substances which 

 result from internal cellular metabolism, the proper excretions 

 of the cell. The secondary products are those which result from 

 the action of a primary product, such as an enzyme, upon some 

 material outside the cell. The distinction is clear enough in 

 theory but practically it is often obscure. 



Enzymes. Fermentation in its broad sense means the 

 chemical changes brought about by living cells or their products. 

 In its more restricted sense, it applies to the splitting of carbohy- 

 drates by the action of microbes, which is accompanied by the 

 evolution of gas. Organisms which cause active fermentation 

 are spoken of as zymogenic. Dextrose, C 6 Hi 2 O 6 , is a readily 

 fermentable carbohydrate and is decomposed in various ways 

 by different microbes. In some instances a large proportion 

 of it is converted into alcohol and carbon dioxide according to 

 the following equation: 



C 6 Hi 2 O 6 (fermented) = 2C 2 H 6 O+2CO 2 . 



Other kinds of micro-organisms produce little alcohol or gas 

 but abundant lactic acid. The reaction may be represented 

 roughly by this equation: 



C 6 Hi 2 6 (fermented) = 2C 3 H 6 O 3 . 



In other instances acetic acid may be produced: 



C 6 Hi 2 O 6 (fermented) =3C 2 H 4 O 2 . 



These equations are only an approximate indication of the 

 reactions which take place, as it is very doubtful that the whole 

 molecule of dextrose is ever converted into a single simpler 

 compound by fermentation, but they will serve to indicate the 



