54 



PATHOGENIC BACTERIA. 



phosphatic salt. Sometimes one pigment is soluble, 

 the other insoluble, so that the colony will appear one 

 color, the medium upon which it grows another. Some 

 organisms will only produce their colors in the light ; 

 others, as the 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 temperature. 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 cult- 

 ure-medium is also of much importance in this connec- 

 tion. Thus, the 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- 

 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 envi- 

 ronment, no matter what the color of the light thrown 

 upon it. 1 1 have found that bacteria which have been 

 robbed of their color by incubation, when placed in the 

 normal environment produce the original color, no mat- 

 ter what color the light they receive. Some of the pig- 

 ments — perhaps most of them — are formed only in the 

 presence of oxygen. 



4. Liquefaction of Gelatin. — When certain forms of 

 bacteria are grown in gelatin the culture-medium is 

 partly or entirely liquefied. This characteristic is en- 

 tirely independent of any other property of the bacte- 

 rium, and is one manifested alike by pathogenic and 

 non-pathogenic individuals. Sternberg and Bitter have 

 shown that if from a culture in which liquefaction has 

 taken place the bacteria be removed by filtration, the 

 filtrate will retain the power of liquefying gelatin, show- 

 ing that the property is not resident in the bacteria, but 

 in some substance in solution in their excreted products. 



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



