PRODUCTS OF METABOLISM. 121 



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 through many 

 generations at high temperatures. Oxygen also is necessary for the 

 chromogenesis of many bacteria. Some need a short exposure to day- 

 light 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, water, alcohol, 



FIG. 48. Bacteriopurpurin, from a Rhodospirillum, crystallized from a chloroform 

 solution. (After Molisch.) 



ether, chloroform, benzol, and other solvents, and according to the change 

 of color caused by acid and alkali (Fig. 48) . A group of carotin bodies, 

 named because of their similarity to the pigment of carrots, the prodigiosin 

 bodies, named after B. prodigiosus, the fluorescent pigments and per- 

 haps a few other groups are distinguished, but their chemical nature is 

 rather vague as yet. The absorption of distinct lines of the spectrum 

 by solutions of these pigments is claimed to be a very reliable means of 

 distinguishing the pigments of different species. 



AROMATIC SUBSTANCES constitute another group of metabolic prod- 

 ucts. The chemical analysis accomplishes more with these compounds 

 than with pigments, since they are frequently well-known compounds. 



