Chapter 5 



Systems of Classification and 



Identification of Groups and 



Species of the Genus 



Streptomyces 



Principles of Separation of Genera 



The historical background and various 

 systems of classification of the order Actino- 

 mycetales in general and of the actinomy- 

 cetes in particular have been discussed in 

 detail in Chapter 4 of Volume I. The princi- 

 ples underlying the generic and specific 

 separation of the organisms are presented 

 in Chapter 4 of the present volume. The 

 variability and overlapping among genera 

 and species have been emphasized in Chap- 

 ter 6 of Volume I. Certain important factors 

 pertaining specifically to the genus Strepto- 

 myces must be considered before any dis- 

 cussion is presented of the separation of this 

 genus into subgenera, series (species-groups), 

 species, and varieties. 



Among the factors that must be empha- 

 sized in any attempt to classify actinomy- 

 cetes, the following three are most impor- 

 tant: (a) the nature of the substrate (or 

 vegetative) growth and the nature of the 

 aerial mycelium, if any; (b) the degree of 

 variability of the cultures; and (c) the effect 

 of the composition of the medium. To facili- 

 tate recognition of the organisms and to 

 establish constant and variable differences 

 foi classification purposes, well-defined 

 media and standard conditions of cultiva- 

 tion must be used. 



Actinomycetes are differentiated from the 

 true bacteria by their filamentous growth 

 and by their true branching. It is often 

 difficult, if not impossible, to distinguish 

 between the profuse branching of certain 

 mycobacteria and the short-lived mycelium 

 of the nocardias, except for the fact that 

 the latter produce mycelium consistently 

 in the early stages of their development. 

 There is a gradual transition between the 

 mycobacteria and the nocardias. It also is 

 often difficult to differentiate between the 

 nocardias and the streptomyces. The latter 

 are characterized by the constant and 

 marked nature of their aerial mycelium, 

 whereas the nocardias are characterized 

 largely by the transitory and undifferenti- 

 ated nature of this mycelium. 



In establishing differences between no- 

 cardias and streptomyces, one must con- 

 sider the following factors: 



I. Nocardias usually have been consid- 

 ered incapable of forming aerial mycelium 

 that could be differentiated from the sub- 

 strate mycelium. It also has usually been 

 assumed that no spirals are ever formed 

 from the mycelium. Recently, however, 

 Cordon and Mihm (1958) have reported 

 that certain nocardias are able to form aerial 

 mycelium similar to thai of streptomyces 



82 



