PRODUCTS OF METABOLISM 1 63 



and bj^ the strong alcohol-producing yeasts of the distillery process. 

 By continued cultivation of an organism upon a certain medium, it 

 will become so acclimatized that it degenerates readily when the con- 

 ditions become unfavorable. Such specifically trained strains of 

 rnicroorganisms are used in alcohoHc and lactic fermentation, in patho- 

 genic bacteriology and in the inoculation of leguminous plants w^th 

 nitrogen-fixing bacteria. 



Products from Nitrogen-free Compounds 



Sugars. — It would be entirely beyond the limits of this book to 

 give an account of all the different ways in which sugars and other 

 compounds can be decomposed by microorganisms. It is much more 

 important, for the beginning bacteriologist, to acquaint himself with 

 the main types of sugar fermentations, and with the characteristics 

 of the organisms which bring about these changes. 



In the action of microorganisms many distinguish somewhat crudely 

 six common types: 



Complete oxidation. 



Partial oxidation. 



Alcoholic fermentation. 



Lactic fermentation. 



Acid gas fermentation. 



Butjnric fermentation. 

 Most of these types have been mentioned previously. 



Complete oxidation of carbohydrates is observed most commonly 

 among molds and mycodermas, and also in a few bacteria, e.g., in Azoto- 

 bacter. It is possible only where, there is a ready oxygen supply, as, 

 e.g., in soils of an open texture, in trickling filters, and on the surface 

 of decaying fruits. 



The incomplete oxidation is, as a rule, more common in nature. 

 Frequently microorganisms produce first an incomplete oxidation, but 

 later oxidize the intermediate products completely. The molds are 

 typical examples. Aspergillus niger is noted for its formation of oxalic 

 acid. If it is grown in a sugar solution, it will bring about at first a 

 rapid increase in acidity, but after a while, it decreases again, when the 

 acid is oxidizing completely. The following processes may be noted: 



CeHiaOs + 9° = sCCOijH)^ -|- 3H2O 



Oxalic acid 



(C02H)a -f- = 2 CO2 -I- H2O 



