NUTRITION OF MICROORGANISMS 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 bacteria 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- 

 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 ; 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. Such treatment sometimes results in the development of special 

 strains or races adapted to the special conditions. 



NUTRITION OF MICROORGANISMS IN 

 NATURAL SUBSTRATES 



It was at first assumed that bacteria and other microorganisms possess 

 a simpler type of metabolism than do higher plants and animals j al- 

 though some can obtain all the nutrients required for cell synthesis and 

 energy from simple elements and compounds, others need for their nu- 

 trition certain highly complicated organic substances. Recently it has 

 been recognized that various "growth-promoting" substances or vita- 

 mins play an important role in the nutrition of many microorganisms. It 

 has also been established that highly complicated enzyme systems are 

 produced by these lower forms of life, and that many interrelationships 

 exist among their metabolic processes, the composition of the medium, 

 and the environmental conditions. One thus begins to realize that the 



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