PHYSIOLOGY OF THE ACTINOMYCETES 361 



red to blue as the reaction of the medium chajiges from acid to al- 

 kaline. This pigment also diffuses into the medium. 



A considerable number of species produce a very characteristic 

 brownish discoloration of the agar. It is very pronounced on potato 

 and gelatin, and easily noticeable in the water-clear peptone solution. 

 At first yellowish, this medium becomes gradually light and then 

 dark brown, and finally almost black. It was formerly believed that 

 the brownish discoloration of the media was characteristic of a single 

 species named A. chromogenus, but it is now known that a consider- 

 able number of species which differ among themselves in other char- 

 acters may form this black pigment. These forms are frequently 

 referred to as the chromogenus group of actinomycetes. 



The formation of this pigment, according to Beijerinck, is due to 

 the action of an enzyme, tyrosinase, on tyrosine, converting this sub- 

 stance to a pigment, melanin; this reaction is supposed to be the 

 same one which causes the spontaneous darkening of potatoes when 

 their cut surface is exposed to the air or when urine, in cases of 

 melanuria, turns dark in color on standing. Skinner -^ found that 

 this pigment was produced only in media which contain free or com- 

 bined tyrosine, and no species produced it in any medium unless it 

 did also in certain synthetic media with tyrosine. Another brown 

 pigment, tinctorially and chemically different, is produced by a few 

 rare species in tyrosine-free media. Another soluble pigment is also 

 produced by some of the actinomycetes from free but not from com- 

 bined tyrosine. This is a pink or red pigment which turns to melanin 

 on prolonged standing or on being heated with sodium hydroxide. 



The pigments produced by the actinomycetes, as was stated, are 

 very striking and, as has been indicated, they are a definite indica- 

 tion of metabolism. However, the biochemical study of pigment pro- 

 duction has only been started. Conn and Conn ^ show that numerous 

 pH indicators other than the well-known red <-> blue indicator of S. 

 coelicolor are produced. The great variability in color produced in 

 different media of the same pH shows that metabolism also is re- 

 sponsible for the kinds of pigments produced. One Streptomyces 

 strain, for instance, was found to produce a bright yellow color only 

 on media containing free or combined histidine. The chemical prop- 

 erties of a few of the pigments have been studied but much more 

 work should be done. 



It is commonly said that pigment production of aerobic actino- 

 mycetes is subject to extreme variation. This statement needs qual- 

 ification. It is true that a strain inoculated into several media of 

 slightly different composition will vary enormously as to color. And 



