SOILS AND WATER BASINS AS CULTURE MEDIA 11 



confined. The synthesis of new cell material by microorganisms is ac- 

 companied by a gain of free energy, which must be supplied by other 

 chemical transformations. Ordinary soils, however, contain microbial 

 nutrients in concentrations sufficient to support a large number of living 

 cells. This can be illustrated by the fact that when a soil is sterilized and 

 then inoculated with a pure culture of bacterium rapid multiplication 

 takes place (Table 3), When fresh water taken from a lake or the sea is 

 kept in the laboratory for one or two days, a great increase in its bac- 

 terial population occurs. 



There is considerable variation in the ease with which a specific or- 



TABLE 3, MULTIPLICATION OF COLIFORM BACTERIA IN STERILE SOIL 



BACTERIA RECOVERED* 



After 10 days After 26 days 



149,000,000 138,000,000 



48,000,000 42,600,000 



1,660,000 240,000,000 



From Waksman and WoodruflF (949). 

 * Per gram of soil. 



ganism can be isolated from a natural substrate and consequently in 

 the techniques employed. Some microorganisms may be present in 

 abundance and can be readily isolated. Others are found only in limited 

 numbers and can be obtained only with considerable difficulty and by 

 the use of special procedures. Still others can be isolated only after the 

 natural substrate is treated in such a manner as to favor the multiplica- 

 tion of the specific organism j this can be done by enriching the soil with 

 a nutrient or substance which the particular organism is able to utilize, 

 or by changing conditions of reaction, by aeration, or by other treat- 

 ment that would favor the rapid development of the organism in ques- 

 tion. Special strains or races of microorganisms may often be developed 

 as a result of such treatment, which tends to favor the adaptation of the 

 organisms present in the soil to a particular process. ■ ^iKl C.37S. 



-.,.jii«^c: 



