88 NUTRITION AND METABOLISM. 



produced in the protoplasm and are either secreted or retained. All pro- 

 ducts of metabolism will be found in the protoplasm of the cell in small 

 quantities. Among other substances frequently found in microorganisms 

 may be mentioned glycogen (C 6 H 10 O 5 ) D which can be readily detected 

 by the brown color it gives when acted upon by iodine. Glycogen may 

 be considered as a reserve substance stored by the organism. In the 

 same way, fats exist in microorganisms, but are more generally found; 

 their presence can be detected by microscopical examinations as well 

 as by chemical tests. The amount of fat in some bacteria is surprisingly 

 high. In the tubercle bacterium 26. to 39.29 per cent of the total 

 solids is fat. All acid-fast bacterial cells have a very high fat content. 

 Other bacteria also contain occasionally as much as 8 per cent fat. 

 Yeasts seem to have a lower fat content, while in molds it has been found 

 to vary from 0.5 to 50.5 per cent. Many other products of organic 

 nature are found occasionally, but their importance is not determined. 

 The minerals of the microbial cell are very essential, and like the 

 organic materials, necessary for the life of the cell. The total ash of 

 bacteria, yeasts, and molds, is small, about 1.5 per cent to 8 per cent of 

 the dry cell. The important minerals which seem necessary for the con- 

 struction of the cell are potassium, calcium, magnesium, iron, manganese, 

 and, of the metalloids, nitrogen, phosphorus, and sulphur. Some other 

 minerals are usually, found, but are unnecessary to the cell, as sodium 

 and silicon. 



AMOUNT OF FOOD REQUIRED. 



The amount of food that is ordinarily decomposed by microorganisms 

 and the amount that is absolutely necessary, differ widely. The quantity 

 of organic and inorganic matter just sufficient to support a very weak 

 growth is certainly very small, since a few species will multiply to some 

 extent in ordinary distilled water. Such water, after having stood for 

 some time, is found to contain several thousand bacteria per c.c. It 

 may seem to the layman that in such water it would be possible to detect 

 easily the organic and inorganic matter of the microorganisms and then 

 it would not be considered distilled water. An estimate of the weight 

 of bacteria demonstrates, however, that this is not the case. If we suppose 

 the average bacterial cell to be a cylinder whose base measures one 

 square micron and whose height is two microns (which is a high estimate) 

 the volume of such a cell would be 1X1X2 cubic microns = o.ooi X 



