Classification 



of the Actinomycetes 



Introduction 



Two important factors must be recognized in any attempt to classify the 

 actinomycetes, especially for the purpose of establishing specific differences. 

 These are, first, the ability of the organisms to produce vegetative and 

 aerial mycelium; and second, the great variability of the cultures. To facili- 

 tate recognition of these organisms and to establish differences for classifi- 

 cation purposes, well-defined media and standard conditions of cultivation 

 must be used. 



The filamentous growth and the true branching of the actinomycetes 

 differentiate these organisms from the true bacteria. 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 nocardias produce mycelium consistently in the early stages 

 of their development. There is thus a gradual transition between the myco- 

 bacteria and the nocardias. There is also a definite transition between the 

 nocardias and the streptomycetes, due to the constant and marked nature 

 of the aerial mycelium of the latter and the transitory and undifferentiated 

 aerial mycelium of the former. 



In differentiating a nocardia from a streptomyces, one must consider the 

 following differences: 1. A nocardia usually does not produce aerial my- 

 celium, and if it does, such mycelium is no different from the vegetative 

 mycelium produced by the organism and never forms spirals; a strepto- 

 myces, on the other hand, produces a characteristic aerial mycelium, a 

 property which may be lost, however, on continued cultivation or under 

 special conditions, and such mycelium frequently forms characteristic 

 spirals. 2. A nocardia multiplies by concentration and segmentation of the 

 protoplasm within a filamentous cell, followed by dissolution of the cell 

 membrane, the fragmented portions of the mycelium usually developing 

 into fresh mycelium under favorable conditions, either by germ tubes or 

 by lateral budding; a streptomyces produces true spores or conidia, the 

 vegetative mycelium not segmenting spontaneously into bacillary or coc- 

 coid forms, but remaining nonseptate and coherent even in old cultures, 

 thus producing the characteristic tough-textured, leathery growth. 3. In 

 a nocardia, the aerial mycelium represents an extension upward of the 

 vegetative mycelium ; it does not exhibit any differentiated protoplasm and 



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