FOOD OF MICROORGANISMS 151 



that the dry matter of 550,000,000 cells of B. coli weigh p.i mg. The 

 amount of food that is used as the building material for the cell is 

 probably larger than the weight of the cell itself, since there will always 

 be present waste products, but it is of the same order of magnitude, i.e., 

 very small and often hardly measurable. The example of the urea fer- 

 mentation (page 146) illustrates this point very well. 



Sources of Carbon. — ^The compounds which can serve as building 

 stones for the cell vary greatly with the species. The source of carbon 

 for all green plants is carbon dioxide, CO2. Animals cannot use this, 

 for they all require complex compounds,- such as carbohydrates, fats 

 or amino-acids. Bacteria exist between the plants and animals in 

 this respect. Some bacteria have already been mentioned (page 147) 

 as being able to use carbon dioxide (CO2), as the only source of carbon; 

 they are the mineral-oxidizing species. Such bacteria are called 

 autotrophic in their relation to carbon, since they use it in the inorganic 

 form. A bacterium feeding on carbon, as such, would be called 

 prototrophic; bacteria of this class are said to exist. The vast majority 

 of microorganisms are heterotrophic, using carbon in organic form. 

 Organic acids and sugars are excellent sources of carbon for micro- 

 organisms, although proteins and their decomposition products seem 

 to be equally satisfactory as construction material. , 



Sources of Nitrogen. — The sources of nitrogen3,re equally varied; 

 the green plants use nitrates; animals must have a number of different 

 amino-acids; the microorganisms again are found between them; We 

 know autotrophic bacteria, and especially molds and yeasts which can 

 grow with nitrates or ammonium salts as the only source of nitrogen. 

 There are three groups of prototrophic bacteria in their relation to 

 nitrogen — the B. amylobacter group, the Ps. radicicola group and the 

 Azotohacter group. These bacteria are of the greatest importance to 

 agriculture; soil fertiUty depends, to a large extent, upon the last two 

 groups, for they take nitrogen gas from the surrounding air, form their 

 own protoplasm from it, and thus increase the amount of chemically 

 combined nitrogen in the soil. Details of their relation to soil fertility 

 can be foimd in Chap. Ill, page 338. The majority of bacteria are 

 heterotrophic, requiring organic nitrogen. Urea is not well adapted for 

 this purpose; amino-acids or the peptones from which amino-acids are 

 derived are the best compounds for most organisms. Asparagin is 

 ^very commonly used if forsome reason peptones are to be omitted. 



