CHAPTER II 



Pure Culture Study and Classification of Soil Bacteria 



It is almost impossible at present to make a complete study of the 

 various types of bacteria occurring in the soil, due both to the great 

 variety of forms and to the lack of sufficient knowledge concerning 

 many of them. Bacteria offer but few stable characteristics which can 

 be utilized for their separation. Classification of bacteria in general 

 and of soil bacteria, in particular, is not fully satisfactory. The different 

 systems of classification can, therefore, be merely tentative; they are 

 based either on the physiological activities of the organisms, especially 

 their carbon, nitrogen, and oxygen metabolism, or on their morphological 

 relationships. In the general subdivision of the soil bacteria, the physio- 

 logical system will be utilized for the greater convenience which it 

 offers in the study of the role of these organisms in the soil; in the 

 differentiation of the individual bacteria within the large groups morpho- 

 logical characters are utilized. 



Pure culture study. In the case of large organisms, a pure culture 

 corresponds to an individual. In the case of the unicellular bacteria, 

 the transformation carried on by one cell is too small, and large num- 

 bers of organisms must take part in a reaction, before measurable 

 changes are obtained; these individual cells, however, must be as much 

 alike as possible. The various bacteria do not exist in the soil in pure 

 culture, but in mixed associations; entirely different results are usually 

 obtained under natural conditions, with the various microbiological 

 processes completing one another, than in pure culture. This is 

 especially true in the case of organisms which depend for their nutrients 

 upon the activities of other organisms, as in the case of the nitrite 

 bacteria which depend upon the various proteolytic organisms for 

 their substrate (ammonia), and the nitrate-forming bacteria which 

 depend upon the nitrite formers for their supply of energy source 

 (nitrite). Antagonistic effects are often marked under natural sur- 

 roundings, while they are eliminated in pure culture. Certain soil 

 processes, such as cellulose decomposition or nitrogen-fixation, are 

 carried on much more actively by crude than by pure cultures of the 

 organisms concerned, due to the fact that the accompanying organisms 



53 



