104 



THE ACTINOMYCETES, Vol. I 



fication of actinomycetes, especially the 

 genus Streptomyces may be taken as an illus- 

 tration. This key is based largely on the 

 pigments produced on organic and synthetic 

 media. On continued cultivation of the cul- 

 tures under artificial laboratory conditions, 

 the pigment may undergo changes in nature 

 and intensity, or frequently be lost alto- 

 gether. When the characters of an organism 

 are based on pigmentation, it becomes very 

 difficult to make comparisons even if type 

 cultures are available. The streptomycin- 

 producing strain of S. griseus could hardly 

 be recognized when compared with the orig- 

 inal description of the organism made by 

 Waksman and Curtis. The latter was based 

 upon cultures that were at first believed to 

 be similar to one described by Krainsky. 

 Since no one ever had an opportunity to 

 compare freshly isolated cultures wdth those 

 of Krainsky, the Waksman and Curtis or- 

 ganism is now usually recognized as the type 

 species for S. griseus. 



S. coelicolor has also been studied exten- 

 sively, especially from the point of view of 

 pigmentation and agar-decomposition. Erik- 

 son observed that the major variations of 

 this organism comprise loss of pigmentation, 

 loss of capacity to produce aerial mycelium, 

 and occasionally loss of ability to liquefy 

 agar. Spontaneous formation of variants 

 could be found more readily in the spores of 

 degenerate colonies, rendered atypical by 

 artificial methods of cultivation. A strain 

 that had lost the power of pigmentation gave 

 a variant which produced sectored colonies, 

 some of which possessed the blue pigment. 



Thomas made a study of the variability 

 of >S'. scabies. He isolated six physiologic races 

 that differed in their pathogenicity on 10 

 different potato varieties. An increase in the 

 nitrogen, phosphorus, and potash content of 

 the medium resulted in a delay in the produc- 

 tion of aerial mycelium. Nitrogen and phos- 

 phorus were generally favorable for growth, 

 but potash tended to retard it. Maximum 



growth and stability of the cultures were 

 obtained on peat soil. Mineral soils tended 

 to retard or inhibit growth and increase 

 variability. The more pathogenic races were 

 most stable on most media. These variations 

 led to the question whether the descriptions 

 of many species as causative agents of potato 

 scab represented distinct species or only 

 variants of a single species. 



In a study of the variability or mutability 

 of .4. mutabilis, an organism apparently be- 

 longing to the streptomycetes, Masumoto 

 showed that in a synthetic medium, with 

 ammonium chloride as a source of nitrogen, 

 the nature of the carbon source greatly in- 

 fluenced the formation of nonaerial myce- 

 lium from aerial mycelium-producing types. 

 Sucrose and lactose were the two sugars most 

 favorable for this purpose. Other carbon 

 sources, notably mannitol, never gave the 

 nonaerial mycelial type. 



Attention has already been directed to the 

 marked variations in cultures of S. griseus 

 (Schatz and Waksman, 1945; Waksman 

 et al., 1948). Formation of aerial mycelium, 

 pigmentation of the substrate mycelium, 

 production of streptomycin, acid formation, 

 glucose consumption, and autolysis were 

 observed among the qualitative and quanti- 

 tative variations. The formation of the vari- 

 ants of S. griseus were described as follows: 



Freshly isolated streptomycin-producing 

 cultures formed typical aerial mycelium, 

 characteristic of the species. These cultures 

 produced an alkaline reaction in glucose- 

 containing media, as well as characteristic 

 surface and submerged types of growth; they 

 underwent only limited lysis and were mark- 

 edly resistant to the antibiotic action of 

 streptomycin. They gave rise to two kinds 

 of variants: 



1. A nonsporulating variant that formed 

 no aerial mycelium and no streptomycin; 

 it was sensitive to the antibiotic action of 

 streptomycin in a glucose-containing me- 

 dium, and was characterized by a type of 



