* 



BIOLOGY OF BACTERIA. 53 



researches show that there are at least three different 

 bacilli whose growth causes the development of poison- 

 ous ptomaines in meat. 



3. Chromogenesis. — Those bacteria which produce col- 

 ored colonies or impart color to the medium in which 

 they grow are called chromogenic ; those with which no 

 color is associated, non-chromogenic. Most chromogenic 

 bacteria are saprophytic and non-pathogenic. Some of 

 the pathogenic forms, as the Staphylococcus pyogenes 

 aureus, are, however, color-producers. It seems likely that 

 the bacteria do not form the actual pigments, but certain 

 chromogenetic substances, which, uniting with constitu- 

 ents of the culture-medium, produce the colors. 



Galleotti l has described two kinds of pigment, one of 

 which, being soluble, readily penetrates all neighboring 

 portions of the culture-medium, like the colors of Bacillus 

 pyocyaneus, and an insoluble pigment which does not 

 tinge the solid culture-media at all, but is constantly 

 found associated with the colonies, like the pigment of 

 Bacillus prodigiosus. The pigments are found in their 

 greatest intensity near the surface of the colony. The 

 coloring matter never occupies the protoplasm of the 

 bacteria (except the Bacillus prodigiosus, in whose cells 

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

 an intercellular excrementitious substance. 



The pigments are so varied as to give almost every 

 known color. It sometimes happens that a bacterium 

 will elaborate two or more colors. The Bacillus pyo- 

 cyaneus thus produces pyocyanin and fluorescin, both 

 being soluble pigments — one blue, the other green. 

 Gessard has shown that when the Bacillus pyocyaneus 

 is cultivated upon white of egg } it produces only the 

 green fluorescent pigment, while in pure peptone solu- 

 tion it grows with the production of blue pyocyanin 

 alone. His experiments prove a very interesting fact, 

 that for the production of fluorescin it is necessary that 

 the culture-medium contain a definite amount of a 



1 La Sperimentale, 1892, xlvi., Fasc. iii., p. 261. 



