304 PRINCIPLES OF SOIL MICROBIOLOGY 



pigment dissolved out into the medium. The aerial mycelium is usually 

 colored white, gray or buff, sometimes red, yellow, brown, light green 

 or bluish-green. The vegetative or substrate growth is usually colored 

 gray, red, yellow, orange, brown or black. Among the soluble pigments, 

 purple, brown, black and yellow are predominant; red, blue and green 

 are also formed by some species. Soluble brown to purple pigments are 

 very common on protein media. A slight difference in the composition 

 of the medium has an important influence upon the pigment formation 

 by actinomyces. 54 



Various attempts have been made to explain the nature of the dark 

 brown pigments produced on organic media. Beijerinck 55 suggested 

 that the brown pigment is a result of quinone-formation, as shown by 

 the fact that ferri-salts color the brown-colored gelatin black and the 

 gelatin itself is made insoluble due to the action of the pigment. In the 

 presence of HC1, iodine is liberated from potassium iodide. The for- 

 mation of the pigment has also been ascribed 56 to the action of an 

 enzyme, as in the case of tyrosine media coloring black. But the organ- 

 isms must be able to synthesize their own tyrosine and produce a brown or 

 dark pigment since the pigment is produced also on tyrosine-free media. 

 Some actinomyces form blue or green pigments, particularly when freshly 

 isolated. The rapidity of gelatin liquefaction and pigment formation are 

 utilized for diagnostic purposes. The characterization of an organism 

 by the nature of the pigments formed has sometimes led to duplications, 

 as in the naming of one organism, on the basis of formation of a red and 

 blue pigment, A. violaceus ruber 57 and A. tricolor. 58 The pigment acts 

 as a natural indicator. On media which are slightly acid (pH 6.0), the 

 pigment is at first red, then, with a change of reaction of the medium to 

 alkaline, the pigment becomes blue. The chemical nature of this pig- 

 ment is yet to be investigated. 



Variability. The actinomyces show, in their morphological and 

 physiological characteristics, greater variability than any other group of 

 organisms. Size, shape and color of colonies, length of mycelium, 

 abundance of mycelium and spore formation are determined largely 



64 Conn, 1921 (p. 290). 



65 Beijerinck, 1900 (p. 299). 



66 Sano, K. Beitrage zur Kenntnis der Oxydasen, insbesondere bei Bak- 

 terien. Diss. Wurzburg. 1902. 



57 Waksman and Curtis, 1916 (p. 290). 



68 Wollenweber, H. W. Der Kartoffelschorf. Arb. d. Forsschungsinst. f. 

 Kartoffelhau. H. 2, Berlin. 1920. 



