618 BACTERIOLOGY OF THE SOIL, WATER, AND AIR 



ments chiefly from inorganic salts. The nitrates are taken up by 

 chlorophyll-bearing plants and, with the energy of sunlight transform 

 them, together with carbon dioxide, water, phosphates and various 

 salts, into the complex vegetable proteins upon which the animal 

 kingdom primarily subsists. 



It is obvious, therefore, that there is a well-defined nitrogen cycle 

 an intricate series of changes which proteins and their derivatives 

 undergo, through which complex, lifeless nitrogenous compounds 

 are reduced through bacterial activity to simple, stable mineralized 

 inorganic combinations of their elements. These elements are 

 restored, chiefly through the synthetic activity of plant life, to the 

 animal kingdom. The nitrogen cycle is, in a sense, a measure of the 

 metabolism of the living earth, in which the anabolic or synthetic 

 processes occur in plants and indirectly in animals; the catabolic or 

 analytic process is brought about chiefly by bacteria. 



In addition to the normal bacterial flora of the soil and adventitious 

 saprophytic organisms, pathogenic bacteria are occasionally found; 

 Bacillus typhosus, dysentery and cholera organisms and other excre- 

 mentitious bacteria are occasionally deposited on the ground with 

 human excrement. These microorganisms do not, as a rule, survive 

 prolonged exposure to air, sunlight and other unfavorable environ- 

 mental vicissitudes, however. Certain spore-forming bacteria Bacil- 

 lus tetani, anthrax, symptomatic anthrax, malignant edema and 

 gas bacilli are very common in certain places. These bacteria, except 

 anthrax, appear to multiply in the intestinal tracts of the herbivora. 



The natural or biological degradation and mineralization of dead 

 organic matter by bacterial activity in the upper layers of the soil, 

 so essential to promote fertility, is of paramount importance in the 

 purification of water and sewage. Indeed, the essential features of 

 the nitrogen cycle are involved in both instances. 



WATER AND SEWAGE. 



The very general distribution of bacteria in the superficial layers 

 of the soil makes it almost inevitable that waters which wash the 

 surface of the earth shall receive some bacteria, consequently rivers 

 and smaller streams, lakes and other surface waters always contain 

 bacteria and other microorganisms. The number of bacteria per 

 unit volume, however, is far less in water than upon the land, unless 

 floods carry large amounts of soil with adherent organisms directly 



