CHAPTER VI. 

 FOOD REQUIREMENTS. 







FOOD is any substance which bacteria can utilize in obtaining 

 either building material or energy for the cell activity. The quan- 

 tity and quality of food necessary vary widely with the different 

 species. However, all foods must contain certain essential ele- 

 ments. Our knowledge at the present time indicates these ele- 

 ments to be carbon, hydrogen, oxygen, phosphorus, potassium, 

 nitrogen, sulphur, calcium, iron and magnesium, or, using the key 

 for remembering as suggested by Dr. Hopkins, we have C. Hopk'ns 

 CaFe Mg - - "C. Hopk'ns Cafe - - mighty good." 



Minimum Requirements. In considering the food used by bacteria 

 a minimum and a maximum requirement must be recognized. 

 These two extremes differ greatly, for although the minimum quan- 

 tities appear inconceivably small the maximum ones are enormous. 

 One may obtain a fair idea of the minimum requirements from the 

 following calculation made by Rahn: '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 

 cubic centimeter. It may seem to the laymen 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, how- 

 ever, that this is not the case. If we suppose the average bacterial 

 cell to be a cylinder whose base measures 1 square micron and 

 whose height is 2 microns (which is a high estimate) . The volume 

 of such a cell would be 1 X 1 X 2 cubic microns = 0.001 X 0.001 

 X 0.002 mm. = 0.000,000,002 cu.mm. The specific gravity of 

 bacteria being very nearly one, the weight of one bacterium would 

 be 0.000,000,002 mg. One hundred thousand cells per cubic centi- 

 meter means 100,000,000 cells per liter, which would weight 0.2 

 mg. Of this total weight, at least four-fifths is water and only one- 

 fifth is solid matter. The total solid matter in 1 liter of water 

 containing 100,000 bacteria per cubic centimeter amounts to the 

 immeasurable quantity of 0.04 mg. Such water will pass the test 

 for distilled water. How much food the bacteria in distilled water 

 have used is impossible to say, since, besides the traces of minerals 



