Chromogenesis 61 



acds and alkalies. Both acids and the alkalies, when in excess, 

 serve to check the further activity of the micro-organisms. 



Chromogenesis. Bacteria that produce colored colonies or impart 

 color to the medium in which they grow are called chromogenic; 

 those producing no color, non-chromo genie. Most chromogenic 

 bacteria are saprophytic and non-pathogenic. Some of the patho- 

 genic forms, as Staphylococcus pyogenes aureus, are, however, color 

 producers. It seems more likely that certain chromogenetic sub- 

 stances unite with constituents of the culture medium to produce the 

 colors than that the bacteria form the actual pigments; but, as Gale- 

 otti* has shown, there are two kinds of pigment, one being soluble, 

 readily saturating the culture medium, as the pyocyanin and fluorescin 

 of Bacillus pyocyaneus, the other insoluble, not tingeing the solid 

 culture media, but retained in the colonies, like the pigment of Bacil- 

 lus prodigiosus. The pigments are found in greatest intensity near 

 the surface of a bacterial mass. The coloring matter never occupies 

 the cytoplasm of the bacteria (except Bacillus prodigiosus, in whose 

 cells occasional pigment-granules may be seen), but occurs as an in- 

 tercellular 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 fluorescin, both being soluble 

 pigments one blue, the other green. Gessardf has shown that 

 when Bacillus pyocyaneus is cultivated upon white of egg, it produces 

 only the green fluorescent 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 fluorescin 

 it is necessary that the culture medium contain a definite amount 

 of a phosphatic salt. Sometimes, an organism produces two pig- 

 ments, one is soluble, the other insoluble, so that the colony will 

 appear one color, the medium upon which it grows another. The 

 author once found an interesting coccus, J with this peculiarity, upon 

 the conjunctiva. It formed a brilliant yellow colony upon the sur- 

 face of agar-agar, but colored the agar-agar itself a beautiful violet. 

 In this case the yellow pigment was insoluble, the violet pigment 

 soluble and diffusible through the jelly. Some organisms will 

 only produce 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, re- 

 fuse to produce pigments at so high a temperature. Thus, Bacillus 

 prodigiosus produces a brilliant red color when growing at the tem- 

 perature 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 



* "Lo Sperimentale," 1892, XLVI, Fasc. in, p. 261. 

 t"Ann. de PInst. Pasteur," 1892, pp. 810-823. 



t See Norris and Oliver, " System of Diseases of the Eye," vol. H, p. 489, and 

 "University Medical Magazine," Philadelphia, Sept., 1895. 



