Assuming these to be cocci of 1µ diameter the volume of these bacteria in a liter is only 0.05 cc. or in the liter there would be 19999 parts of milk and only 1 part bacteria. The surface area of these bacteria is 3141.6 sq. cm. With this large surface exposed, it is not strange that the change from “on the point of souring” to “sour” occurs within an hour or less.
Although large numbers of bacteria can and do cause great chemical changes the amount of material actually utilized for maintenance of the cell is very slight, infinitesimal almost, and yet is fairly comparable to that required for man, as is illustrated by the following computations:
E. Kohn has shown that certain water bacteria grew well in water to which there was added per liter 0.000002 mg. dextrose, 0.00000007 mg. (NH4)2SO4 and 0.0000000007 mg. (NH4)2HPO4. The bacteria numbered about 1000 per cc. Taking the specific gravity at 1 (a little too low) the mass of the bacteria in the liter was about 0.001 mg. Hence the bacteria used 0.002 of their weight of carbohydrate and 0.00007 of ammonium sulphate. A 150-pound (75-kilo) man can live on 375 g. of sugar (0.005 of his weight) and 52.5 g. of protein (0.0007 of his weight). From these figures it can be calculated that the man utilizes about two and a half times as much carbohydrate and about seven times as much nitrogen as the bacterium, relatively speaking.
CHAPTER IX.
PHYSIOLOGICAL ACTIVITIES.
The physiological activities of motion, reproduction and metabolism within the cell have been discussed in previous chapters. The objects in view in the discussion of the “physiological activities” (sometimes spoken of as “biochemical” activities) of bacteria in this and subsequent chapters are to familiarize the student to some extent with the great range of chemical changes brought about by these minute organisms, to show their usefulness, even their necessity, and to impress the fact that it is chiefly by a careful study of these “activities” that individual kinds of bacteria are identified. It should always be borne in mind that the bacteria, in bringing about these changes which are so characteristic in many instances, are simply engaged in their own life struggle, in securing the elements which they need for growth, in liberating energy for vital processes, or occasionally in providing conditions which favor their own development and hinder that of their competitors.
FERMENTATION OF CARBOHYDRATES.
By this is meant the changes which different carbohydrates undergo when subjected to bacterial action.[11]
These changes are marked chiefly by the production of gas or acid. The former is called “gaseous fermentation” the latter “acid fermentation.” The gases commonly produced are carbon dioxide (CO2) hydrogen and marsh gas (CH4). Other gases of the paraffin series may also be formed as ethane (C2H6), acetylene (C2H2), etc. CO2 and H are the ones usually formed from sugars by the few gas-forming bacteria which produce disease, though even here some CH4 is present. The common Bacterium coli forms all three, though the CH4 is in smallest quantity.
