PRODUCTS OF MICROBIAL ACTIVITIES 233 



Kruse has called attention to the fact that these complex equations 

 can well be explained as the simultaneous occurrence of the following 

 simple fermentations: 



C 6 H 12 O 6 = 2H 2 + 2 CO 2 + CH 3 CH 2 CH 2 CO 2 H 



C 6 H 12 O 6 = 3CH 3 CO 2 H 



C 6 H 12 O 6 + 6H 2 O = 6CO 2 + i2H 2 



The first fermentation continues when the others have already ceased, 

 and thus the last stage of Perdrix's equations is very simple. Brede- 

 mann also found that the proportion of the various products formed by 

 B. amylobacter varies greatly with the conditions, and the same has been 

 recently established in the fermentation of B. coll. 



Other complications occur when an organism is able to use its own 

 products as food, as is the case with some acetic bacteria. They will 

 at first produce considerable amounts of acetic acid and after a while 

 they oxidize the acid completely. It becomes impossible to account for 

 microbial activity by a chemical equation when several organic com- 

 pounds are decomposed at the same time as is found to occur in some 

 foods, as butter, cheese, ensilage and in sewage. It is also impossible 

 to formulate exactly decompositions which are caused by mixed cultures. 

 The complications become so great and the relations between different 

 organisms are so little known that it is useless to make the attempt. 



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 microbiologist 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. 



Butyric fermentation. 

 Most of these types have been mentioned previously. 



