54 PRINCIPLES OF SOIL MICROBIOLOGY 



either use up the deleterious waste products or in some other way- 

 contribute to the process in question. 



To be able to identify the various species of bacteria and determine 

 their specific functions upon nutritive media and in the soil, it is neces- 

 sary to obtain them in pure culture. A mere microscopic examination 

 of bacteria is usually insufficient for their identification. They should 

 be studied on artificial culture media and records made of the cultural 

 characteristics and biochemical changes produced. Among these 

 characteristics, we may include size, appearance and color of colony; 

 growth upon solid and liquid media; modification of color, consistency, 

 reaction and chemical composition of the medium; action upon sugars 

 and proteins and enzyme formation, and influence of oxygen tension, 

 temperature and chemical agents. Among the morphological charac- 

 teristics, we may include form, size, grouping and appearance of cells, 

 motility, spore formation, manner of reproduction, etc. 



Pure cultures of bacteria and other microorganisms are often weak- 

 ened, when grown on artificial culture media, or may lose altogether 

 their capacity of producing the change which they carry on in nature, 

 as in the case of certain nitrogen-fixing bacteria. This leads to differ- 

 ences in results in the hands of different investigators. We are dealing 

 here with organisms whose functions are changeable and which are very 

 variable in their activities. 1 It is best to work with freshly isolated 

 organisms and it is always advisable to establish the constancy of 

 physiological characteristics. 



In obtaining pure cultures of a number of soil bacteria, especially 

 those producing certain biochemical changes when grown in the presence 

 of specific substances, the selective or accumulative culture method used 

 extensively by Beijerinck, 2 Winogradsky and others may be utilized. 

 The first step made in the isolation of a particular organism consists in 

 adding some of the material (soil or manure) to a liquid medium con- 

 taining nutrients especially adapted to the growth of that particular 

 organism. This allows the accumulation of the organism in question 

 and eliminates largely the other accompanying forms. By transferring 

 repeatedly to the same sterile medium, an enrichment culture is ob- 

 tained. Theoretically one species is obtained under ideal enrichment 



1 Pringsheim, H. Die Variabilitiit niederer Organismen. Berlin. J. 

 Springer. 1910. 



2 Richter, O. Die Bedeutung der Reinkultur. Borntraeger. Berlin. 1907; 

 Stockhausen. Oekologie, Anhaufungen nach Beijerinck. 1907. 



