BIOLOGICAL FORMATION OF METHANE 



dans, 11 Mbact. sohngenii, 7 Ms. methanica, Mc. mazei) have 

 been purified by isolation and reisolation of colonies from 

 agar media until they were free of other species of methane 

 bacteria, but they may still have been contaminated by 

 other anaerobic bacteria. The complete or partial purifica- 

 tion of these eight species indicates that the isolation of 

 pure cultures of other species should also be possible. It 

 must be emphasized, however, that the isolation of methane 

 bacteria is generally a difficult and time-consuming task 

 requiring great patience and considerable experience with 

 anaerobic culture techniques. So far, pure cultures have 

 been obtained in only three laboratories. Even after a pure 

 culture has been isolated its preservation is no easy problem. 

 Most of the pure cultures have been lost after a few years. 

 Only one species, Mbac. omelianskii, has been kept in pure 

 culture for a long period of time. 



Because of the difficulties encountered in the isolation 

 and maintenance of pure cultures of methane bacteria only 

 a rather limited amount of information has been obtained 

 concerning the biological and physiological characteristics 

 of the group as a whole. Nevertheless some tentative gen- 

 eralizations can be made that are useful in defining the 

 group and indicating the probable behavior of its members 

 under a variety of circumstances. 



Relation to Oxygen. All of the species are strictly 

 anaerobic; they develop only in the absence of oxygen and 

 in the presence of a suitable reducing agent. 7 The methane 

 bacteria are much more sensitive to oxygen or certain other 

 oxidizing agents, such as nitrate, than are most other an- 

 aerobic bacteria. For this reason they can be grown more 

 easily in liquid or semisolid media than on the surface 

 of an agar medium. Even with liquid media not fully pro- 

 tected from air, sufficient oxygen may leak in to inhibit 

 cultures that have passed the peak of their activity. The 



