BACTERIAL FERMENTATIONS 



abundant constituent of plants it was reasonable to sup- 

 pose that it must be a major source of methane in decom- 

 posing plant materials. Furthermore, methane had been 

 shown to be formed in large amounts in the digestive tracts 

 of herbivorous animals; therefore the possible role of 

 methane-producing bacteria in cellulose digestion was of 

 some concern to animal physiologists. 



The experiments of Popoff, 4 Tappeiner, and Hoppe- 

 Seyler 5 on the utilization of cellulose by crude enrichment 

 cultures of bacteria obtained from soil or the digestive 

 tracts of herbivorous animals demonstrated that cellulose 

 is in fact decomposed under anaerobic conditions, fre- 

 quently with the formation of methane and other products 

 including carbon dioxide, hydrogen, and acetic and butyric 

 acids. At first this was taken to mean that the bacteria 

 which attacked cellulose also form methane. However it 

 was soon realized that another interpretation is possible, 

 namely, that the methane is formed not by the cellulose- 

 decomposing bacteria but by the action of other associated 

 microorganisms on one or more of the products of the 

 cellulose fermentation. This interpretation was supported 

 by two lines of evidence. First, certain cellulose-fermenting 

 cultures were found to produce carbon dioxide and hydro- 

 gen but no methane. This proved that cellulose fermenta- 

 tion is not necessarily associated with methane formation. 

 Second, Hoppe-Seyler, Omelianski, and particularly Sohn- 

 gen 1 demonstrated that the products of cellulose fermenta- 

 tion, such as formate, acetate, butyrate, ethanol, and even 

 hydrogen and carbon dioxide, can be readily used as sub- 

 strates by methane-producing bacteria. 



Although these results were consistent with the conversion 

 of cellulose to methane by a two-stage process, they did not 

 exclude the existence of methane-producing bacteria that 

 attack cellulose. Omelianski (1895-1904) 6 tried to resolve 



