llftcrotiophic liacteria 71 



In addition to the bacteria, other groups of microorganisms also 

 contain thermophihc forms. This is indicated by such names as 

 Thcrmonujces, Thcrmoactinoimjces, and Actinomyces thermophilits. 



Myxobacteria. Myxobacteria occur abundantly in manure and 

 in soil. The total number of these organisms depends upon the 

 nature of the soil. Some are found only in alkaline, neutral, or 

 faintly acid (pH 8.0-6.0) soils, others in very acid soils (pH 3.7-5.9), 

 and still others are independent of the reaction. Moist soils are more 

 favorable for their development than dry soils; cultivation of soil has 

 also a fa\orable effect. Peat bogs and moist forest soils contain a 

 specific flora of myxobacteria. 



To demonstrate their presence, balls of rabbit manure, previously 

 moistened with water and sterilized, are placed on the surface of 

 the soil. As many as 7-10 species may thus be obtained from a 

 single soil sample. Certain myxobacteria can be isolated by the use 

 of living fungus mycelium, such as VerticiUium, growing in a dish; 

 the bacteria cause the destruction of the mycelium. 



Some of the myxobacteria play an important role in the decompo- 

 sition of vegetable residues in soil, notably the cellulose. It has even 

 been suggested that the very active cellulose-decomposing group 

 Cytophaga represents a group of myxobacteria (Myxococcus). 



Denitrifying Bacteria. A large number of microorganisms are 

 able to reduce nitrates to nitrites or to ammonia. Only specific bac- 

 teria, however, can reduce, under certain conditions, the nitrate to 

 elementary nitrogen and to oxides of nitrogen, which can thus escape 

 into the atmosphere. Under anaerobic conditions, the nitrate may 

 serve as a source of oxygen for these bacteria, with organic carbon 

 compounds as sources of energy. This process is usually referred to 

 as complete denitiification, and the bacteria are spoken of as denitri- 

 fying bacteria. 



Various denitrifying bacteria have been isolated from horse 

 manure, cattle excreta, and soil. Van Iterson demonstrated the 

 presence in soil of organisms designated as B. stutzeri, B. denitro- 

 fluorescens, and B. viilpinus. The same soil which favors nitrifica- 

 tion under aerobic conditions will favor denitrification in absence of 

 free oxygen. 



Several organisms reducing nitrates are capable of obtaining their 

 energy from inorganic compounds. ThiohaciUus denitrificans, an 

 organism widely distributed in the soil, oxidizes sulfur and reduces 

 nitrate to nitrogen gas. Thiosulfate can be oxidized under anaerobic 

 conditions only in the presence of nitrate as a source of oxygen. The 



