FOOD OF MICROORGANISMS. 89 



o.ooi Xo.oo2 mm. = 0.000, 000,002 cu. mm. The specific gravity of bac- 

 teria 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 one liter of water containing 100,000 bacteria per c.c. amounts to the 

 unmeasurable 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 (CO 2 ), ammonia (NH 3 ), hy- 

 drogen (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 i per cent of 

 lactic acid, because this is the highest acid concentration that this bacterium 

 can endure. If this acid is neutralized, the inhibiting 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 meta- 

 bolism of the cell (p. 147). 



ORGANIC FOOD MATERIALS. 



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. The solid matter 

 of 250,000,000 bacteria will amount to about o.i mg. 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 be waste products, but it is of the same order of magni- 

 tude, i.e., very small and often hardly measurable. The example of 

 the urea fermentation (on p. 85) illustrates this point very well. 



