176 NUTRITION AND METABOiISM 



the mycelium. Pigments cannot be considered as reserve substances, 

 since many pigments are excreted and remain outside the colorless 

 cells. Pigment production may be, incidental. It is possible that the 

 waste products of certain organisms happen to be colored. 



After Beyerinck, the chromogenic bacteria may be divided into three 

 classes: 



1. Chromophorous bacteria, in which the pigment is placed in the cell 

 and has a certain biological significance analogous to the chlorophyl 

 of higher plants. In this division belong the green bacteria discovered 

 by Van Tieghem and Engelmann and the red sulphur bacteria or purple 

 bacteria. ' 



2. Chromoparous or true pigment-forming bacteria, which set free the 

 pigment as a useless excretion, either as a color-body or as a leuco-body 

 which becomes colored through the action of atmospheric oxygen. The 

 individuals themselves are colorless and may under certain conditions 

 cease to form pigments. To this class belong B. prodigiosus, B. cyano- 

 genes, Ps. pyocyanea, and others. 



3. Parachrome bacteria, which form the pigment as an excretory prod- 

 uct but retain it within their bodies, as B. janthinus and B. violaceus. 



. When the pigment is soluble in water, as those produced by Ps. 

 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 pn 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 

 through many generations at high temperatures. Oxygen also is 

 necessary for the chromogenesis of many bacteria. Sonle 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 chedk pigment production in the same degree as do 

 antiseptics and other harmful influences. 



