246 NUTRITION AND METABOLISM 



far as enzymes and toxins are concerned, it is not even determined 

 whether or not they are of protein nature. The last two groups are 

 known only by their actions, while the pigments are very conspicuous 

 and cannot possibly be overlooked. 



PIGMENTS have naturally attracted the attention of microbiologists 

 ever since pure cultures were known, and many investigators have tried 

 to explain the nature and the meaning of pigments. All experiments 

 concerning the purpose of pigment-formation by microorganisms have 

 been without results. It is not known that the pigment is of any 

 material advantage to bacteria; for it is possible to cultivate colorless 

 strains of pigment bacteria which grow apparently as well as the original 

 pigmented culture. Again, pigments cannot take the place of the 

 chlorophyl in plants except perhaps the bacteriopurpurin of the purple 

 bacteria. It does not even protect the cells against intense light, 

 because the pigmented organisms are not more resistant than the corre- 

 sponding colorless "sports." The only exception are the colored spores 

 of the molds, especially. Penicillium and Aspergillus, which are very 

 resistant to light, while the spores of Oidium are killed just as easily as 

 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, 



