Chromogenesis ^ 53 



pyocyaneus, the other insoluble, not tingeing the solid cul- 

 ture media, but retained in the colonies, like the pigment 

 of Bacillus prodigiosus. The pigments are found in great- 

 est intensity near the surface of a bacterial mass. The 

 coloring matter never occupies the cytoplasm of the bac- 

 teria (except Bacillus prodigiosus, in whose cells occasional 

 pigment-granules may be seen), but occurs as an inter- 

 cellular deposit. 



Almost all known colors are formed by different bacteria. 

 One bacterium will sometimes elaborate two or more colors ; 

 thus, Bacillus pyocyaneus produces pyocyanin and fluores- 

 cin, both being soluble pigments one blue, the other green. 

 Gessard* has shown that when Bacillus pyocyaneus is culti- 

 vated upon white of egg, it produces only the green fluor- 

 escent pigment, but if cultivated in pure peptone solution 

 it produces only the blue pyocyanin. His experiments prove 

 the very interesting fact that for the production. of fluor- 

 escin it is necessary that the culture medium contain a defi- 

 nite amount of a phosphatic salt. Sometimes, when an 

 organism produces two pigments, one is soluble, the 

 other insoluble, so that the colony will appear one 

 color, the medium upon which it grows another. I once 

 found an interesting coccus, t with this peculiarity, upon 

 the conjunctiva. It formed a brilliant yellow colony 

 upon the surface of agar-agar, but colored the agar-agar 

 itself a beautiful violet. In this case the yellow pig- 

 ment was insoluble, the violet pigment very soluble and 

 diffusible through the jelly. Some organisms will only pro- 

 duce pigments in the light; others, as Bacillus mycoides 

 roseus, only in the dark. Some produce them only at the 

 room temperature, but, though growing luxuriantly in the 

 incubator, refuse to produce pigments at so high a tem- 

 perature. Thus, Bacillus prodigiosus produces a brilliant 

 red color when growing at the temperature of the room, 

 but is colorless when grown in the incubator. The reaction 

 of the culture medium is also of much importance in this 

 connection. Thus, Bacillus prodigiosus produces an intense 

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

 colorless or pinkish upon slightly acid media. Colored 

 lights seem to have no modifying influence upon the pig- 



* " Ann. de 1'Inst. Pasteur," 1892, pp. 810-823. 



t " See Norris and Oliver, " System of Diseases of the Eye," vol. n, p. 

 489, and " University Medical Magazine," Sept., 1895. 



