140 PHYSIOLOGICAL CHEMISTRY. 



C. BACTERIOLYTIC PROCESSES. 



The term bacteriolytic is applied to such fermentation- 

 splitting processes as may be carried out by bacteria without 

 the addition of oxygen. In the acetic acid fermentation, 

 which is likewise a bacterial process, the presence of oxygen 

 is necessary, but there are several somewhat analogous reac- 

 tions in which oxygen is not required and these are included 

 in the present group. It must be admitted of course that the 

 division is a perfectly artificial one based on convenience rather 

 than on marked differences in agents or products. Some of 

 the reactions classed here have long been described as fermen- 

 tations and have been studied in connection with the other 

 common ferment changes. These will be taken up first. But 

 we have, in addition, further changes which are certainly of 

 the same general character and call for like treatment. 



LACTIC AND BUTYRIC FERMENTATIONS. 



Why milk turns sour spontaneously in warm weather is an 

 old question, but it was not satisfactorily answered until after 

 the time of Pasteur's pioneer labors. Following his work on 

 the yeasts Pasteur took up other problems of fermentation and 

 pointed out the general nature of the reaction by which the 

 sour substance present, lactic acid, is formed. He found the 

 production of lactic acid to depend on the ferment activity of 

 certain microorganisms, which have later been more fully 

 described by bacteriologists. 



Lactic Acid Bacteria. It was found that lactic acid is 

 formed from the simple sugars by a splitting process which 

 for a long time was illustrated by an equation supposed to 

 represent the facts quantitatively : 



It was also recognized that not merely one, but many species 

 of bacteria are capable of decomposing sugar solutions in this 

 way. Of these the form known as Bacillus acidi lactici has 

 been perhaps the most thoroughly studied; it appears to be 

 always present in milk which has soured spontaneously, and 



