222 NUTRITION AND METABOLISM 



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 liter, which would weigh 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 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 like carbon monoxide (CO), 

 carbon dioxide (02), ammonia (NH 3 ), hydrogen (H), and perhaps 

 methane (CH 4 ). Under all circumstances the amount of food used is 

 very small. 



On 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 will 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 

 decomposed 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 254). 



FOOD FOR 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 

 that the dry matter of 550,000,000 cells of B. coli weigh o.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 202) 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 



