166 PRINCIPLES OF SOIL MICROBIOLOGY 



the separation of spore-forming anaerobes from other spore-forming 

 anaerobes. The improper separation has led to exaggerated claims for 

 the nature and activities of the organisms. All or some of the following 

 procedures are utilized for this separation: 



1. Heating the soil so as to kill the vegetative forms, then introducing 

 various diutions of the heated soil suspension into the proper medium 

 and making transfers from the culture at different stages of develop- 

 ment (heating the culture every time a new transfer is made). The 

 various spore-forming anaerobes sporulate at different periods of their 

 development: some, like the saccharolytic bacteria, sporulate early; 

 others, like most proteolytic forms, sporulating late. 



2. Use of selective media stimulating the predominant development 

 of the organism sought. This method has been of great help in the 

 isolation of some important soil anaerobes. It is sufficient to mention 

 that by the use of selective media and proper environmental conditions, 

 such organisms as the anaerobic nitrogen-fixing forms, thermophilic and 

 cellulose-decomposing forms and others were isolated. The specific 

 medium is inoculated with an infusion of soil or manure, which may 

 be previously heated, if the organism in question forms spores, and 

 incubated at the desired temperature. The adjustment of the medium 

 to specific reactions may often be sufficient to separate one group of 

 organisms from another, often even anaerobic forms from one another. 

 For instance, the adjustment of the nitrogen-free glucose media to a 

 pH of 5.5 will not only favor the development of the nitrogen-fixing 

 Clostridium pastorianum, but will also prevent the development of the 

 proteolytic organisms, which usually accompany it. 22 For the enrich- 

 ment of cellulose decomposing anaerobic organisms, the use of a specific 

 liquid medium or of a silica gel plate with cellulose as the only source of 

 energy is recommended (p. 196). For the decomposisition of hemicel- 

 lulose, physiological salt solution containing cubes of potato has been 

 used, 23 while, for starch splitting organisms, media containing 1 per 

 cent peptone broth and 5 per cent starch have been suggested. 24 



3. The use of aniline dyes for the elimination of certain species of 

 organisms. 



4. Selective temperatures for the enrichment of various organisms, 



22 Dorner, 1924 (p. 165). 



23 Ankersmit, P. Untersuchungen iiber die Bakterien im Verdauungskanal 

 des Rindes. Centrbl. Bakt. I, Orig, 39: 359-574, 687. 1905; 40: 100-118. 



24 Choukevitch, J. Etude de la flore bacte>ienne du gros intestin du cheval. 

 Ann. Inst. Past. 25: 247. 1911. 



