PLATE METHOD FOR COUNTING MICROBES 



53 



sible. When a small portion of this suspension is added to the 

 medium which has been brought to the Uquid condition (by warm- 

 ing), mixed well and the medium then allowed on cooling to solidify- 

 in Petri dishes, the individual cells of the various organisms 

 become fixed in separate positions on the dish. If the medium 

 contains all of the nutrients required for the growth of these 

 microbes and if other conditions, such as reaction, temperature, 

 and circulation of gases are favorable, the organisms multiply 



Fig. 39. — Azotobacter cells in soil, as demonstrated by direct staining 

 (from Winogradsky). 



rapidly, with the result that colonies are formed from each of the 

 original cells present in the soil suspension. After incubation for a 

 few hours or days, the colonies become visible to the naked eye, 

 and their total number can be determined by simple enumeration 

 (Fig. 40). If the dilution has been so conducted that a known 

 amount of soil was added to the medium, one can calculate, from 

 the number of visible colonies which appear on the plate, the 

 number of organisms per unit of soil which are able to develop 

 under this particular set of conditions. 



The plate method possesses several distinct advantages. 

 Determinations can be made of the abundance in the soil of such 



