CHAPTER III 

 FOOD OF MICROORGANISMS* 



MOISTURE REQUIREMENT 



Moisture may be called the most important factor of life. Not 

 only bacteria, but every microscopic and macroscopic being requires a 

 considerable amount of moisture. Living organisms contain on the 

 average between 70 per cent and 90 per cent of water, and only 10 per 

 cent to 30 per cent of solid matter. Microorganisms which live 

 entirely submerged in liquids need water not only within but without 

 the cells. Bacteria, yeasts, molds, and some protozoa obtain their food 

 by diffusion through the cell-membrane; their food-substances must 

 be soluble and dissolved. No other liquid can take the place of water. 



The amount of water required by microorganisms cannot be stated 

 briefly. Several factors have to be taken into consideration, as the 

 osmotic pressure, the insoluble and the colloidal substances, the species 

 of organisms, temperature, and perhaps others. (See pp. 184, 203.) 



AMOUNT OF FOOD REQUIRED 



The amount of food that is ordinarily decomposed by microorgan- 

 isms 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 so that it could 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 i square micron and whose height is 2 

 microns (which is a high estimate) the volume of such a cell would be 

 1X1X2 cubic microns = o.ooi X o.ooi X 0.002 mm. = o.ooo,- 



* Prepared by Otto Rahn. 



221 



