52 Biology of Bacteria 



with acids simultaneously formed. Some bacteria produce 

 acids only, some alkalies only, others both acids and alka- 

 lies. Both acids and the alkalies, when in excess, serve 

 to check the further activity of the micro-organisms. 



6. Chromogenesis. Bacteria that produce colored colo- 

 nies 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 pathogenic forms, as Staphylo- 

 coccus pyogenes aureus, are, however, color producers. It 

 seems more likely that certain chromogenetic substances 

 unite with constituents of the culture medium to pro- 

 duce the colors than that the bacteria form the actual 

 pigments; but, as Galeotti * 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, 1 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 excrementitious substance. 



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 f has shown that when Bacillus pyocyaneus is culti- 

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

 escent pigment, but if cultbdM^ in pure peptone solution 

 it produces only the blue^jjg^BPSin. 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, J with this peculiarity, upon 



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

 t"Ann. del'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. 



