370 MICROBIOLOGY OF SOIL 



Among the rod-shaped species B. mycoides, B. subtilis, B. mesen- 

 teriCus, B. tumescens and other members of the subtilis group are quite 

 prominent. Members of the amylobacter group are seldom absent. 

 Members of the proteus group and various fluorescens are always 

 present, while Bad. cerogenes and allied species are common inhabitants 

 of the soil. 



(Physiological Groups). In the decomposition of organic matter in 

 the soil certain important changes in both nitrogenous and non-nitro- 

 genous material are accomplished by definite groups of bacteria. The 

 breaking down of protein substances is accomplished by the forma- 

 tion of ammonia, nitrites and nitrates. These in turn may be trans- 

 formed back into more complex amino-compounds, peptones, and pro- 

 teins, or they may be destroyed with the evolution of free nitrogen. 

 Moreover, there are groups of bacteria capable of joining non-nitro- 

 genous organic matter to elementary nitrogen and of producing thus 

 nitrogen compounds. Again, there are groups of bacteria bearing 

 distinct and important relations to the decomposition of cellulose, or 

 the transformation of its cleavage products, methane and hydrogen. 

 There are, likewise, definite groups of bacteria concerned in the 

 transformation of sulphur and its compounds, and of ferrous compounds. 



METHODS or STUDY 



METHODS FOR COUNTING BACTERIA. There are two methods for 

 the quantitative determination of bacteria in the soil: the plate method 

 and the direct count method. By the use of the plate method we can 

 obtain only relative results, since not all soil bacteria are able to grow 

 and develop into colonies even on the most suitable media. The plate 

 method shows cells of bacteria that are able to develop under laboratory 

 conditions but furnishes no direct evidence as to their exact number. 

 Conn therefore suggested the direct count method, already employed suc- 

 cessfully in the bacteriological examination of milk. A smear is prepared 

 by spreading o.i c.c. of the soil infusion over an area of i sq. cm., then 

 stained with Rose Bengal in carbolic acid. The bacteria are colored 

 deep pink or red, while the mineral particles remain uncolored and most 

 of the organic matter is unstained or stained yellow or light pink. 

 The bacteria are then counted by means of an oil-immersion objective 

 and a high power eye-piece. The actual numbers of bacteria detected 



