62 Biology of Micro-organisms 



scarlet-red color upon alkaline and neutral media, but is colorless 

 or pinkish upon slightly acid media. Some of the pigments— per^ 

 haps most of them— are formed only in the presence of oxygen. 



Production of Odors. — Gases, such as H2S and NH4, and acids, 

 but5rric and acetic acids, have sufficiently characteristic odors. 

 There are, however, a considerable number of pungent odors which 

 seem to arise from independent odoriferous principles. Many of 

 them are extremely unpleasant, as that of the tetanus bacillus. ^ The 

 odors seem to be peculiar individual characteristics of the organisms. 



Production of Phosphorescence. — Cultures of Bacillus phos- 

 phorescens and numerous other organisms are distinctly phosphor- 

 escent. So much light is sometimes given out by gelatin cultures 

 of these bacteria as to enable one to see the face of a watch in a 

 dark room. Gorham found the photogenesis most marked when 

 the organisms are grown in alkaline media at room temperature. 

 Most of the phosphorescent bacteria are found in sea-water, and are 

 best cultivated in sea-water gelatin. Some are famiUar to butchers 

 through the phosphorescence they cause on the surface of stale 

 meats. 



Production of Aromatics. — Phenol, kresol, hydrochinone, hydro- 

 paracumaric 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 

 nitrosoindol reaction. To perform it, 10 cc. of the fluid to be tested 

 receive an addition of 10 drops of concentrated sulphuric acid. The 

 mixture is shaken in a test-tube. A few cubic centimeters of a 0.02 

 per cent, solution of potassium nitrite are then allowed to flow down 

 the side of the tube. If indol is present, a purple-red color develops 

 at the junction of the two fluids. f McFarland and Smallf 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. 



The Formation of Nitrates. — A process of fundamental 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 putrefactive changes have occurred, ap- 

 pears as ammonia. The odor of this gas is often plainly perceptible 



* " Zeitschritt. f. physiol. Chemie," viii, p. 417. 



t See Grubs and Francis, "Bull, of the Hyg. Laboratory," 1902, No. 7. 



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



