176 GENEEAL BIOLOGY OF MICRO-ORGANISMS 



into by some of the chemical products of growth. Almost all 

 the other important practical effects of the growth of micro- 

 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 melaboUsm, 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 sphtting of carbohydrates 

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

 of gas. Organisms which cause active fermentation are spoken of 

 as Zymogenic. Dextrose, C6H12O6, 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 alcphol and carbon dioxide according to the following 

 equation : 



CeHijOeCfermented) = 2C2H6O + 2CO2. 



Other kinds of micro-organisms produce Uttle alcohol or gas 

 but abundant lactic acid. The reaction may be represented 

 roughly by this equation : 



C6Hi206(fermented) = 2C3H6O3. ^ 



In other instances acetic acid may be produced : 



C6Hi206(fermented) = 3C2H4O2. 



These equations are only an approximate indication of the re- 

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

 molecule of dextrose is ever converted into a single simpler 



