PRODUCTS OF MICROBIAL ACTIVITIEP 



247 



pyocyanea and the fluorescent bacteria, it diffuses through the medium. 

 When the pigment is not soluble, it either lies within the cell wall or 

 between the individuals. 



This classification furnishes some details concerning the methods of 

 pigment production, which depends upon the presence of certain media. 

 According to Sullivan, sometimes certain mineral salts, sometimes sugar 

 will stimulate chromogenesis. The same is true with molds. Very 

 brilliant colors appear with certain species of molds if grown on cellu- 

 lose or on fat, while on gelatin the pigment is not produced. The tem- 

 perature is an important factor. A large number of chromogens 

 produce no pigment when grown in the incubator. It is possible to 

 obtain non-pigmentation with many species by propagating them 



FIG. 113. Bacteriopurpin, from a Rhodospirillum, crystallized from a chloroform 



solution. (After Molisch.} 



through many generations at high temperatures. Oxygen also is 

 necessary for the chromogenesis of many bacteria. Some need a short 

 exposure to daylight in order to produce their pigment, while cultures 

 grown in absolute darkness may remain colorless. Strong sunlight, 

 however, will check pigment production in the same degree as do 

 antiseptics and other harmful influences. 



The chemical nature of microbial pigments is little known. They 

 are distinguished according to the solubility in various liquids, as water, 

 alcohol, ether, chloroform, benzol, and other solvents, and according 

 to the change of color caused by acid and alkali. A group of 

 carotin bodies, named because of their similarity to the pigment 

 of carrots, the prodigiosin bodies, named after B. prodigiosus, the 



