BACTERIAL FERMENTATIONS 



beneficial effect on the growth of methane bacteria caused 

 by the addition of solid sediments, such as diatomaceous 

 earth and shredded asbestos, to a liquid medium may be 

 attributed at least partially to a mechanical shielding of the 

 bacteria from dissolved oxygen, although other explana- 

 tions are possible. 



The most satisfactory reducing agents for pure cultures 

 are generally sodium sulfide and hydrosulfite. Care must 

 be used in adjusting the concentration of these substances 

 since above the optimal level they become toxic to some 

 species. Mylroie and Hungate 12 have recently described 

 the successful use of hydrogen and palladium chloride as 

 a reducing system in media for Mbact. formicicum. The 

 effectiveness of this reducing system supports the conclusion 

 that a low oxidation-reduction potential in the medium is 

 more important than the presence of a specific concentra- 

 tion of a reduced sulfur compound per se. 



Energy Metabolism. A second characteristic of methane 

 bacteria is their energy metabolism which is specialized for 

 a process that produces methane as a major product. A 

 little later we shall take a closer look at the nature of the 

 energy-yielding reactions. Now it is sufficient to empha- 

 size that the ability to form methane is not a common and 

 widely distributed property of anaerobic bacteria but is 

 restricted to a specialized group. A few possible excep- 

 tions to this generalization are recorded in the literature, 

 particularly in the work of Laigret. 13 This investigator 

 reported that Clostridium perfringens, which normally does 

 not produce methane, can be induced to do so in a peptone- 

 formate medium by the addition of a small amount of 

 iodine. However, because of the absence of experimental 

 details, the validity of Laigret's observations is difficult to 

 assess. A thorough reinvestigation of this problem is nec- 

 essary before a definite conclusion can be reached concern- 



