28 SELMAN A. WAKSMAN 



SUMMARY 



1. The actinomycetes do not fix any atmospheric nitrogen, 

 although some colonies will develop on routine nitrogen free 

 media. 



2. Most species are able to reduce nitrates to nitrites with the 

 proper source of carbon, a few species are able to reduce nitrates 

 to nitrites actively with nearly all sources of carbon studied, 

 while a few others give no reduction or only traces with nearly 

 all sources of carbon. 



3. The proteins and amino acids studied were found to form 

 the best sources of nitrogen for this group of organisms. Amides 

 are used only to a very small extent. Nitrates are used fairly 

 well in the presence of the proper source of carbon. Nitrites 

 present in small quantities in the medium are utilized well by 

 most species, particularly by those that reduce nitrates actively. 

 Ammonium salts form the poorest sources of nitrogen, with 

 glycerol as a source of carbon; with glucose as a source of carbon 

 both amides and ammonium salts are utilized well as sources of 

 nitrogen, if the reaction of the medium does not tend to become 

 too acid. 



4. Most actinomycetes split proteins actively as indicated by 

 an increase of the amino-nitrogen content of the medium. The 

 organisms that produce only a small amount of growth split 

 proteins only to a very limited extent and use up only small 

 quantities of the amino acids. 



5. The production of ammonia from proteins and amino 

 acids is not characteristic of this group, although, on continued 

 incubation, considerable quantities of ammonia may accumulate 

 in the medium, as indicated by the growth of the organisms in 

 milk or on pure proteins added to sterilized soil. 



6. Many species produce soluble yellow, brown to dark brown 

 pigments in media containing proteins and amino acids, the 

 production of a brown pigment being due, in most cases, not to 

 a tyrosinase reaction. Only some strains of A. scabies and a 

 few other chromogenus species are able to produce a soluble 

 brown pigment from tyrosin; most of the species that produce 



