THE CHEMISTEY OF BACTERIA. 95 



solution, may produce an equally typical putrefaction when 

 allowed to develop on albuminous matter. The distinction 

 between saprogenic and zymogenic bacteria is, therefore, not 

 very marked. Furthermore, well known pathogenic bac- 

 teria may give rise to a typical fermentation, or putrefac- 

 tion according to the nature of the material on which they 

 are grown. 



While it is customary to speak of alcohol, acetic, buty- 

 ric, lactic acid, etc., fermentations, it must not be supposed 

 that each one of these fermentations is due to a single spe- 

 cific organism. It will presently be shown that alcoholic 

 fermentation may be caused by a large number of species of 

 yeasts, by many moulds and even by bacteria. Similarly, 

 acetic acid fermentation is brought about by any one of a 

 large number of species. The same is true of the other 

 common fermentations. 



The chemical changes observed in fermentations may 

 result in one of several ways. By far the most common 

 change is that of hydration. In this case, the elements of 

 water are introduced into the molecule which then breaks 

 up. This is seen in the following equations which repre- 

 rent the inversion of cane-sugar, and the fermentation of 

 urea. 



C12H2.2O,, + H2O = CijHijOe + CoHijOe. 

 Cane-sugar. Dextrose. Levulose. 



COCNHj)^ + HjO = CO, + 2 NH,. 



Urea. 



A second way in which cleavage may result is by 

 halving. Thus, in lactic acid fermentation, glucose is appar- 

 ently changed direct into two molecules of lactic acid. 



C6H,208 ^ 2 C2H6O3. 



Glucose. Lactic acid. 



A third and very important change observed in certain 

 fermentations is that of oxidation. The best illustration of 



