56 Biology of Bacteria 



the use of a nitrate broth made by dissolving in 1000 c.c. 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 : 



II. Sulphanilic acid, 0.5 gram. 



Hydric acetate, diluted, 150.0 c.c. 



Keep the solutions in glass-stoppered bottles and mix equal 

 parts for use at the time of employment. 



About 3 c.c. of the culture and an equal quantity of the 

 uninoculated culture fluid are placed in test-tubes and about 

 2 c.c. of the test fluid slowly added to each. The develop- 

 ment 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 pro- 

 duced by the growth of the bacteria. 



The presence of ammonia in either fluid is easily deter- 

 mined by the immediate development of a yellow color or 

 precipitate when a few drops of Nessler's solution f are 

 added. 



Failure to determine either ammonia or nitrites may 

 not mean that the nitrates were not reduced, but that they 

 were reduced to N. It is, therefore, necessary to test the 

 solutions for nitrates, which is done by the use of phenol- 

 sulphonic acid and sodium hydroxid, which in the presence 

 of nitrates give a yellow color. 



Combination of Nitrogen. Not only do bacteria de- 

 stroy or reduce nitrogen compounds, but some of them 



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



f Nessler's solution consists of potassium iodid, 5 grams, dissolved 

 in hot water, 5 c.c. Add mercuric chlorid, 2.5 grams, dissolved in 10 

 c.c. of water, then to the mixture add potassium hydrate, 16 grams, 

 dissolved in water, 40 c.c., and dilute the whole to 1000 c.c. 



