54 , Biology of Bacteria 



ment-production. Even if for successive generations the 

 bacterium be grown so as to be colorless, it speedily recovers 

 its primitive color when restored to its old environment. I * 

 have found that Bacillus prodigiosus, robbed of its color 

 for many generations by incubation, when placed in the 

 normal environment produces its original red pigment, no 

 matter what color the light thrown upon it. Some of the 

 pigments perhaps most of them are formed only in the 

 presence of oxygen. 



Production of Odors. Gases, such as H 2 S and NH 4 , and 

 acids, butyric and acetic acids, have sufficiently character- 

 istic odors. There are, however, a considerable number of 

 pungent odors which seem to arise from independent odor- 

 iferous principles. Many of them are extremely un- 

 pleasant, as that of the tetanus bacillus. The odors seem 

 to be peculiar individual characteristics of the organisms. 



Production of Phosphorescence. Cultures of Bacillus 

 phosphorescens and numerous other organisms are dis- 

 tinctly phosphorescent. 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 phos- 

 phorescent bacteria are found in sea-water, and are best 

 cultivated in sea-water gelatin. Some are familiar to 

 butchers through the phosphorescence they cause on the 

 surface of fresh 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 nitroso- 

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

 tested receive an addition of 10 drops of concentrated sul- 

 phuric acid. The mixture is shaken in a test-tube. A 

 few cubic centimeters of a 0.02 per cent, solution of potas- 

 sium 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 Small J 



* " University Medical Magazine," July, 1894, vol. vi, No. 10, p. 675. 

 f See Grubs and Francis, " Bull, of theHyg. Laboratory," No. 7, 1902. 

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



