THE ENVIKONMENT OF BACABEIA. 63 



appropriate these elements from inorganic sulphates and 

 phosphates. The various chlorides found in nature furnish 

 the necessary chlorine, and incidentally such metals like 

 sodium and potassium. The traces of calcium, iron, and 

 other metals present in various liquids are sufficient, as a 

 rule, to meet the requirements of an organism. 



The amount of mineral constituents necessary to sup- 

 port bacterial life is exceedingly small. As shown above, 

 the ash constitutes only about one per cent, of the growing 

 bacteria. It has been estimated that 1 mg. of living bacteria 

 contains about 30,000,000,000 cells, in which case this large 

 number of organism will yield only lU mg. of ash. Inasmuch 

 as the organic constituents of bacteria make up less than 

 15 per cent, of the whole it will be seen that the amount of 

 organic matter necessary as food is likewise exceedingly 

 small. This is shown by the fact that many bacteria can 

 multiply not only in ordinary water, but even in distilled 

 water. 



The reaction of the nutrient medium exerts an important 

 influence upon the growth of bacteria. An acid reaction is 

 not as favorable as an alkaline one. Thus, an acidity cor- 

 responding to 20 or 30 c. c. of normal acid per liter will inhibit 

 the growth of many bacteria, whereas the same organisms 

 will thrive in a medium, the alkalinity of which corresponds 

 to 50 c. c. of normal alkali per liter. It is customary, there- 

 fore, to cultivate bacteria upon a neutral or slightly alka- 

 line soil. The moulds, on the other hand, seem to thrive 

 best upon an acid medium. 



Organic matter derived from dead animals or plants can 

 be met with almost everywhere. The simple food require- 

 ments of bacteria are, therefore, widely distributed in nature 

 and for that reason these organisms will be found almost 

 universally present upon the surface of the globe. There 

 are but few places where bacteria are absent. The air at high 



