66 Biology of Micro-organisms 



further process of oxidation is carried on by the nitrifying bacteria 

 whereby nitrates are ultimately formed. These are eminentlj 

 adapted for use by plants, and so the soil is rendered continuously 

 capable of supporting vegetation. 



Nitrosomonas and Nitrosococcus convert ammonia into nitrous 

 acid, and Nitrobacter oxidizes the latter to form nitric acid. 



These genera are well nigh universal in the soil. They do not 

 grow on the ordinary culture media, but require special solutions, 

 free from the diffusive albumins — ^free, indeed, from organic com- 

 pounds of any sort. Their supplies of carbon are obtained by the 

 dissocf ation of carbon dioxid. It is highly noteworthy that they are 

 thus able to flourish without food more complex than ammonia, a 

 fact which is without parallel among organisms devoid of chlorophyl. 



Reduction of Nitrates. — A considerable number of bacteria are 

 able to reduce nitrogen compounds in the soil or in culture media, 

 prepared for them, into ammonia. To the horticulturist this matter 

 is of much interest. Winogradsky* has described specific nitrifying 

 bacilli which he found in soil, and asserts that the presence of ordi- 

 nary bacteria in the soil causes no formation of nitrites so long as the 

 special bacilli are withheld. 



Reduction of nitrates can be determined experimentally by the 

 use of a nitrate broth, made by dissolving in looo cc. of water i gram 

 of peptone and 0.2 gram of potassium nitrate. The ingredients are 

 dissolved, filtered, then filled into tubes, and sterilized. The tubes; 

 are inoculated and the results noted. As nitrites and ammonia are, 

 however, commonly present in the air and are taken up by fluids, it is 

 always well to control the test by an uninoculated tube tested with 

 the reagents in the same manner as the culture. 



Two solutions are employed | for testing the culture: 



I. Naphthylamin, p.i gram, f Boil, cool, filter, and add 156 cc. of 



Distilled water, 20.0 grams, \ dilute (i : 16) hydric acetate. 



II. Sulphanilic acid, 0.5 gram. 



Hydric acetate, diluted, 150.0 cc. 



Keep the solutions in glass-stoppered bottles and mix equal parts 

 for use at the time of employment. 



About 3 cc. of the culture and an equal quantity of the uninocu- 

 lated culture fluid are placed in test-tubes and about 2 cc. of the 

 test fluid slowly added to each. The development of a red color 

 indicates the presence of nitrites, the intensity of the color being in 

 proportion to the quantity of nitrites present. If a very slight 

 pinkish or reddish color in the uninoculated culture fluid and a deeper 

 red in the culture develop, it shows that a small amount of nitrites 

 was already present, but that more have been produced by the 

 growth of the bacteria. 



* "Ann. de I'lnst. Pasteur," 1891; "La Semaine m6dicale," 1892. 

 t "Journal of the American Public Health Association," 1888, p. 92. 



