gin green, central mass covered by gray aerial mycelium. Potato agar: Fair growth, partly sub- merged, covered with grayish white aerial mycelium; medium becomes discolored. Blood agar: Heavilj^ textured small drab colonies, aerial mycelium microscopical; no hemolysis. Dorset's egg medium: Large, round, colorless, scale-like colonies, radially wrinkled; growth brownish, medium dis- colored in 2 weeks. Serum agar: Smooth colorless discoid colonies; marked umbilication after 2 weeks. Broth: Large fluffy white hemispherical colonies, loosely coherent. Synthetic sucrose solution: A few large round white colonies with smooth partly zonate margins, lightly coherent in sedi- ment; later smaller colonies in suspension attached to side of tube. Milk: Coagulation; one-third peptonized. Carrot plug: Colorless raised colonies with powdery white aerial mycelium; after 1 month, very much piled up, aerial myce- lium gray; after 2 months, superabundant growth around back of plug, confluent, greatly buckled, all-over gray aerial my- celium. Antagonistic properties : Positive. Source: Isolated from a case of strepto- thricosis of liver (Willmore, Trans. Roy. Soc. Trop. Med. Hyg., 17, 1924, 344). Habitat: Unknown. 150. Sterile (non-conidia -forming) .species. In view of the fact that various species of Streptomyces are able to lose the capacity to produce aerial mycelium, either on con- tinued cultivation or by a sort of mutation, cultures are obtained which may be mis- taken for nocardias. They can be recognized, however, by the structure of their vegeta- tive mycelium and by their cultural and physiological properties, such as forma- tion of soluble pigments, liquefaction of gelatin, hydrolysis of starch, inversion of sucrose, coagulation and peptonization of milk. Occasionally some are able to revert to the typical streptomycete condition or to regain the capacity to produce aerial mycelium. Such cultures represent many species. Their growth is moi'e commonly colorless, but sometimes pigmented, smooth or lich- enoid, leathery, compact, with shiny surface. Some produce a soluble brown pigment. This was recognized by Krassilnikov, who desig- nated such cultures as Actinomyces albus sterilis and A. viridis sterilis, similar to the formation of Fungus sterilis. He isolated from the soil about 100 such cultures. These were divided into three groups: 1. Strongly proteolytic cultures capable of liquefying gelatin in 3 to 5 days, of pep- tonizing milk in 6 to 10 days, with or without preliminary coagulation, of hydrolyzing starch with varying degrees of rapidity, of inverting sugar. No growth on cellulose. Strongly antagonistic. 2. Gelatin slowly liquefied, in 15 to 30 days, or not at all in that time; milk coagu- lated and peptonized simultaneously; starch hydrolyzed with varying degrees of rapidity or not at all. No growth on cellu- lose. Weak antagonistic properties. 3. Milk coagulated, due to acidification, but not peptonized. No antagonistic effects. Waksman (in Waksman and Lechevalier, Actinomycetes and Their Antibiotics, Balti- more, 1953, 20) has used for these groups such names as Streptomyces sterilis albus, Streptomyces sterilis ruber, Streptomyces sterilis viridis, Streptomyces sterilis flavus, etc. to designate variant forms of cultures which have lost the capacity to produce aerial mycelium. In his collection many of the cultures that originally produced aerial mycelium have lost this capacity and could, therefore, no longer be considered as typi- cal. For example Streptomyces griseus, a vigorously growing culture capable of pro- ducing streptomycin, yielded a mutant which no longer produces aerial mycelium, nor is it able to produce streptomycin. On the other hand, certain nocardia-like organisms have been isolated from natural substrates which, on continued cultivation on artificial media, gave rise to variants which produced sporulating aerial hyphae. This is true, for example, of the culture designated by Gause as Proactinomyces cyaneus-antibioticus and thought to be iden-
