368 ACTINOMYCETES 



Some of these organisms which secrete fat-splitting enzymes have 

 been found to grow in butter with a definite production of acid, re- 

 sulting in rancidity. Streptomyces have been found growing on 

 rubber and decomposing it very slowly. 



It is not possible to identify most of the species' of Streptomyces 

 which are isolated. Waksman's " keys and species descriptions, 

 published several years ago, are the best available. This material 

 has been condensed in a key in Bergey's Manual, Editions 1 to 5 in- 

 clusive, and in Waksman's Principles of Soil Microbiology.^^ The 

 identification is based very largely on cultural characters and more 

 particularly on pigmentation on media of precise composition. For 

 these the student is referred to AVaksman's monographic treatment ^"^ 

 of the group. After using Waksman's methods several years, one of 

 the authors is compelled to admit that he has been unable to identify 

 more than a small fraction of hundreds of isolates he has obtained 

 from soil. The fault lies not in the very careful and excellent descrip- 

 tion of Waksman, nor, it is believed, in the technique used, but in 

 the fact that there are hundreds of "species" or "varieties" in soil 

 and the chance of a person's isolating one that Waksman isolated is 

 small. The most one can usually do is to place an isolate as "close 

 to this or that" species. Actually identification of species is not an 

 end in itself, and if one can know that he has a saprophytic Strepto- 

 myces species, he may w^ell be satisfied and leave specific identifica- 

 tion to specialists at the present stage of our knowledge of this group. 

 Stanier's^^ finding with the variants of Streptomyces species that 

 certain nutritional characteristics are much more stable characters 

 than pigment production is very suggestive for further work on classi- 

 fication of this group, and probably a complete reinvestigation is in 

 order. The question of the validity of the species concept for this 

 group has been posed and, although we are not willing to go so far 

 as to agree that Streptomyces cannot be effectively classified into 

 species, possibly as much could be said for abandoning the species 

 concept for this genus as for any group of microorganisms. 



LITERATURE 



The older literature on the taxonomy, morphology, and physiology of the 

 actinomycetes will be found by consulting the monographs •'• "• ^^< ^^ men- 

 tioned in the chapter. We have listed only the recent papers to which we have 

 referred, and have omitted the older citations found in Henrici's first edition 

 unless, because of bibliography or textual materials, there was special reason 

 for their inclusion. It is felt that the material contained in these earlier 



