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BIOLOGY OF BACTERIA. 51 



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 

 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. Colored lights seem to 

 have no modifying influence upon the pigment-produc- 

 tion. 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 environment, no 

 matter what the color of the light thrown upon it. Bac- 

 teria which have been robbed of their color by incuba- 

 tion, when placed in the normal environment produce 

 the original color, no matter what color the light they 

 receive. Some of the pigments 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. 

 These products are described as u tryptic enzymes" by 



