15° NUTRITION AND METABOLISM 



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 

 I X I X 2 cubic microns = o.boi X olooi X 0.002 mm. = 0.000,- 

 000,002 cu. mm. The specific gravity of bacteria being very nearly i, 

 the weight of one bacterium would be 0.000,000,002 mg.; 100,000 cells 

 per c.c. means 100,000,000 cells per Uter, which would weigh 0.2 mg. 

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

 soUd matter. The total solid matter in i liter of water containing 

 100,000 bacteria per c.c. amounts to the immeasurable quantity of 

 0.04 mg. Such water will pass the tests for distilled water. How 

 much food the bacteria in distilled water have used is impossible to say, 

 since besides the traces of minerals in the water, they obtain some food 

 from volatile compounds of the air Kke carbon monoxide (CO), 

 carbon dioxide (CO2), ammonia (NH3), hydrogen (H), and perhaps 

 methane (CH4). Under all circumstances the amount of food used is 

 very small. 



Oni the other extreme, the maximum amount of food cannot be 

 stated very definitely. Usually bacteria cease to cause decomposition 

 because of the accumulation of noxious metabolic products. The 

 ordinary bacterium from sour milk wiU not form more than about one 

 per cent of lactic acid, because this is the highest acid concentration 

 that this bacterium can endure. If this acid is neutralized; the in- 

 hibiting cause is removed, and the lactic fermentation starts anew 

 until the maximum acidity is reached again. The amount of food 

 detomposed depends largely upon the power of the organism to resist 

 its own products. If the food is too concentrated, however, physical 

 influences may interfere with the metabolism of the cell (page 179). 



Food foe Growth 



The total weight of a large bacterial cell is estimated in the pre- 

 ceding paragraph to be about 0.000,000,002 mg., of which only about 

 one-fifth is dry matter. The smallest quantity that can be weighed 

 accurately on ordinary analytical balances is o.i mg. This corre- 

 sponds to about 250,000,000 bacteria. MacNeal and associates found 



