NUMBERS OF MICROORGANISMS 27 



X 2 = = S (x - x) 2 , 



X 



where x is the mean and x any individual number of colonies counted 

 on a plate; S stands for summation and X 2 as the index of variability. 



With a carefully improved technic, an accurate conformity with the 

 theoretical distribution can be attained even with a mixed bacterial 

 flora of the soil. Any significant departure from the theoretical dis- 

 tribution is a sign that the mean may be wholly unreliable. 



Co?nparison of plate and microscopic methods. By comparing the 

 results obtained by the microscopic and plate methods of determining 

 the numbers of microorganisms in the soil, Conn 48 came to the conclu- 

 sion that if an organism that can grow well on the plate is present in a 

 soil, the plate count will be nearly as high or even higher than the micro- 

 scopic count. In the case of large-sized bacteria, like Bac. cereus, the 

 microscopic count was found by Conn to be higher than the plate count ; 

 in the case of small bacteria, like Bact. fluorescens, or the orange-lique- 

 fying type, which are easily overlooked under the microscope, the plate 

 count is considerably higher. For the determination of the aerobic, 

 heterotrophic, non-nitrogen fixing bacteria in the soil, which develop 

 readily on the plate, the latter method is as satisfactory as the micro- 

 scopic method. A number of bacteria, like the cellulose-decomposing 

 forms, some urea bacteria, and others, even among the aerobic hetero- 

 trophic organisms, are not capable of developing on the common plate, 

 while some nitrogen-fixing bacteria may develop. The distinct differ- 

 ence between plate and microscopic counts of bacteria in normal soil 

 is due not only to the organisms which are not capable of growing on 

 common nutrient media, but also to the fact that a large number of 

 cells, even of the organisms capable of growing on the common plate, 

 do not develop into colonies. The actual number of bacteria in the 

 soil is probably five to twenty and even more times as high as the 

 counts obtained by the plate method. 



For the present, however, the plate method is the most satisfactory 

 one for determining the relative abundance of microorganisms (bac- 

 teria, actinomyces and fungi) in the soil. For determining the num- 

 bers of protozoa and specific physiological groups of bacteria, the dilu- 

 tion method still remains the most satisfactory. In the case of actino- 

 myces and especially fungi, a colony on the plate represents either a 

 spore or a piece of mycelium in the soil; since the spore is only the 



48 Conn, 1918 (p. 7). 



