34 BACTERIAL FERMENTATIONS 



the fermentation can proceed according to reaction 5. Actu- 

 ally the stoichiometric relations are not quite so simple 

 because some hydrogen gas is evolved; consequently some- 

 what less than 1 mole of acetate is consumed per mole of 

 lactate. 



Acetate is usually required in butyric acid fermentations 

 of lactate, glycerol, and other compounds more reduced than 

 carbohydrate. Whenever the ability of an organism to 

 ferment such compounds is to be tested acetate should be 

 supplied in the medium. In the past, failure to do this has 

 resulted in erroneous conclusions concerning substrate utili- 

 zation and the taxonomic relations of certain butyric acid 

 bacteria. 



Although acetate is usually required for the butyric acid 

 fermentation of lactate, at least one exception to this gen- 

 eralization is known. Butyribacterium rettgeri readily fer- 

 ments lactate in the absence of added acetate; in fact both 

 acetate and butyrate are produced. 13 This is possible be- 

 cause B. rettgeri possesses an additional oxidative mech- 

 anism, by which part of the carbon dioxide it forms is 

 reduced to acetate. 14 - 15 This type of oxidative mechanism 

 occurs in several anaerobic bacteria. 16 ' 17 ' 18 



Use of Enzyme Preparations. Most of the early work 

 on the chemistry of butyric acid fermentations was done 

 with growing cultures or washed cell suspensions. Experi- 

 ments with such material provided information concerning 

 the over-all chemical changes in these fermentations and 

 permitted some rough conclusions about the course of the 

 metabolic pathways. However, it eventually became appar- 

 ent that a complete elucidation of the fermentation reac- 

 tions could only be achieved by the use of cell-free extracts 

 and purified enzyme systems. 



The first extensive use of cell-free extracts in the study 

 of the butyric acid fermentation was made by Koepsell and 



