Autotrophic Bacteria 65 



Sexeral typos of nitrite-forming organisms are found in various 

 soils. These bacteria were classified by Winogradsky into four 

 groups: 



1. Nitrosonionas. Free, motile forms, present in the soil as cocci 

 or as rods with rounded ends. Optimimi reaction is at pH 8.6-8.8; 

 some strains may ha\e their optimum at /jH 9.1-9.2, and others at 

 pH 7.5-7.8; growth ceases at pH 6.0. 



2. Nitrosocystis. Masses of cocci surrounded by a membrane. 

 Optimum pH 7.4-7.8. 



3. Nitrosospira. Spiral-shaped forms. 



4. Nitrosogloca. Zoogloea-producing organisms. 



Fig. 28. Nitrate-forming bacterium, Nitrobacter sp. (from Fred and Davenport). 



Not all the various types of nitrite-forming bacteria occur in all 

 soils; the last group is found, for example, only in uncultivated soils. 

 They differ greatly in their activity, the Nitrosomonas being most 

 active and the Nitrosospira least. The Nitrosocystis is found in for- 

 est soils, including both mull and raw-humus soils. 



The numbers of the nitrifying bacteria per gram of soil vary 

 greatly, from a few cells to as many as 24,000. The dilution method 

 is commonly used for this determination. In view of the fact, how- 

 ever, that many cells are usually added to a liquid medium before 

 growth can take place, since the artificial conditions of culture are 

 not so favorable for their development as in normal soil, the actual 

 number of living cells in the soil is far greater than indicated by this 

 method. In humid soils, the nitrifying bacteria are present in the 



