54 Biology of Bacteria 



butchers through the phosphorescence they cause on the 

 surface of fresh meats. 



9. Production of Aromatics. Phenol, kresol, hydro- 

 chinone, hydroparacumaric acid, and paroxyphenylic-acetic 

 acid are by no means uncommon products of bacteria. The 

 most important is indol, which was at one time thought to 

 be peculiar to the cholera spirillum, but is now known to 

 be produced by many other bacteria. The best method of 

 testing for it is that of Salkowski, known as the nitroso- 

 indol reaction. To perform it, 10 c.c. of the fluid to be tested 

 receive an addition of i c.c. of a 0.02 per cent, solution of 

 potassium nitrite. The mixture is shaken in a test-tube. 

 A few drops of concentrated sulphuric acid are then allowed 

 to flow down the side of the tube and form a layer at the 

 bottom. If indol is present, a purple-red color develops 

 at the junction of the two fluids. McFarland and Small* 

 have found that the intensity of this color corresponds to 

 the quantity of indol present, and that quantitative tests 

 can be made by means of a comparative color test series. 



10. The Formation of Nitrates. A process of funda- 

 mental importance is carried on by certain lowly bacteria 

 of the soil. Since plants are unable to assimilate the free 

 nitrogen of the air, but must obtain this element from 

 the soil in the form of some soluble compound, and since 

 there is a relatively limited amount of combined nitrogen 

 in the world, it becomes of the last importance that the 

 supplies which are continually withdrawn from the soil 

 should be replaced by the nitrogen liberated in the decay 

 of organic material. This nitrogen, after a series of putre- 

 factive changes have occurred, appears as ammonia. The 

 odor of this gas is often plainly perceptible about manure 

 heaps. In this form nitrogen is poorly adapted for use by 

 plants, and moreover may be easily dissipated. An ex- 

 tensive further process of oxidation is carried on by the 

 nitrifying bacteria, whereby nitrates are ultimately formed. 

 These are eminently adapted for use by plants, and so the 

 soil is rendered continuously capable of supporting vege- 

 tation. 



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 

 * "Trans, of the American Public Health Association," 1905. 



