24 



THE MICROBE AND ITS ACTIVITIES 



Fig. 10. — Aerobic nitrogen-fixing bac- 

 terium, Azotobacter chroococcum (from 

 Krzemieniewski) . 



they have frequently been referred to as Protista (first hfe). All 

 these microbes have one thing in common, namely, they are all 



very small, and, with the ex- 

 ception of the higher animal 

 forms, such as earthworms and 

 larvae, and the fruiting bodies 

 of mushrooms and certain 

 other fungi, they cannot be 

 seen with the naked eye. 



Various methods have been 

 developed for the study of soil 

 microbes. These include (1) 

 microscopic methods for the 

 study of the organisms as 

 found in the soil; (2) cultural 

 methods for the study of the 

 activities of the microbes in 

 the soil itself; (3) methods for 

 the enumeration of different 

 groups of soil microbes; (4) methods for the isolation and culti- 

 vation in pure culture of 

 various microbes. 



In order to understand 

 the role of a certain microbe 

 in a known soil process, it 

 is desirable to isolate this 

 microbe from the soil, 

 grow it in pure culture on 

 artificial media, then re- 

 inoculate it into sterile soil 

 and bring about the proc- 

 ess in question under con- 

 trolled conditions. A pure 

 culture of a microbe is an 

 isolated cell which repro- 

 duces a number of times 

 giving identical cells. Such 

 a culture allows an ex- 

 aggeration of the activities of the particular microbe, so as to 

 make accessible to observation the elementary principles of cell 



Fig. 11. — Anaerobic nitrogen-fixing bac- 

 terium, Clostridium pastorianum (from Omeli- 

 ansky and Solounskoff ) . 



