12 BACTERIAL FERMENTATIONS 



cializes in the oxidation of C 4 to C 6 fatty acids to acetate 

 and propionate. 11 Finally, Ms. barkerii is restricted to the 

 decomposition of hydrogen, carbon monoxide, methanol, 

 and acetate. 9 This species does not attack longer chain 

 alcohols or fatty acids. 



Since all the available evidence indicates that these species 

 are representative of the group, it is apparent that methane 

 bacteria show a rather extreme substrate specificity. This 

 implies that several species of methane bacteria must be 

 required for the complete methane fermentation of the 

 variety of compounds that are present in sewage and 

 decomposing plant materials. Even for the complete fer- 

 mentation of so simple a compound as valeric acid, as many 

 as three species of methane bacteria may be required. 

 Valerate is oxidized by Mbact. suboxydans to acetate and 

 propionate, which are not further attacked by this or- 

 ganism. 11 Methane is formed in a coupled reduction 

 reaction. A second species, Mbact. propionicum, converts 

 the propionate to acetate, carbon dioxide, and methane. 

 Since this organism cannot attack acetate, a third species 

 such as Mc. mazei is required to ferment the acetate. The 

 need to establish a balanced population of bacteria to 

 participate in this type of fermentation sequence undoubt- 

 edly explains why considerable time is always required to 

 develop a culture capable of causing a rapid and complete 

 fermentation of complex mixtures of organic compounds. 

 When such a culture is obtained, it is capable of main- 

 taining itself more or less indefinitely when a fresh supply 

 of organic materials is added continually, because the major 

 products of the fermentation are gases which escape from 

 the medium leaving behind very little in the way of poten- 

 tially toxic byproducts. 



Nutrition. The general nutritional requirements of 

 methane bacteria appear to be very simple, although this 



