C IT A P T K K 1 



Historical Background 



What Are Actinoniycetes? 



Actinomycetes are a group of branching 

 unicellular organisms, which reproduce either 

 b}' fission or by means of special spores or 

 conidia. They are closely related to the true 

 bacteria; frequently, they are considered as 

 higher, filamentous bacteria. They usually 

 form a mycelium which may be of a single 

 kind, designated as substrate (vegetative), 

 or of two kinds, substrate (vegetative) and 

 aerial (in part sporogenous) . 



In the early descriptions, actinomycetes 

 were often defined as "unicellular microor- 

 ganisms, 1 n in diameter, filamentous; 

 branching monopodial, seldom dichotomous, 

 producing colonies of radiating structure." 

 Two forms of reproduction have commonlj^ 

 been recognized: (a) fragmentation, or oidia- 

 formation, and (b) segmentation. Both kinds 

 of spores grow in ordinary media to form a 

 filamentous mycelium. 



Frequently the actinomycetes have been 

 looked upon as a separate group of organisms 

 occupying a position between the filamen- 

 tous fungi and the true bacteria. It has even 

 been said that actinomycetes are the original 

 prototypes from which both fungi and bac- 

 teria have been derived. Some forms of 

 actinomycetes, such as members of the 

 genus Nocardia, are known to have their 

 counterparts among the l)acteria; other 

 forms, like some species of Strepto7nijces, Mi- 

 cromonospora, and some of the other genera, 

 have their counterparts among the fungi. 

 The similaritv in diameter between bacteria 



and the mycelium and spores of actinomy- 

 cetes and certain common chemical and bio- 

 chemical properties, recently discovered, sug- 

 gest that the actinomycetes should be 

 classified with the bacteria. They are usually 

 placed in a separate order, the Actincnnyce- 

 tales, which is said to be distinct from the 

 Eubacteriales, or the true bacteria, although 

 this relationship has recently been ques- 

 tioned. 



The actinomycetes are generally recog- 

 nized to represent a large and heterogeneous 

 group of microorganisms, comprising several 

 genera and numerous species. They vary 

 greatly in their morphology, physiology, 

 biochemical activities, and role in natural 

 processes. They play an important part in 

 the cycle of life in nature by bringing about 

 the decomposition of complex plant and ani- 

 mal residues and the liberation of a continu- 

 ous stream of available elements, notably 

 carbon and nitrogen, essential for fresh plant 

 growth. Some of the biochemical activities 

 of the actinomycetes are now being utilized 

 for the large-scale production of chemical 

 substances essential for public health and 

 human economy. 



Early Concepts 



The early history of the actinomycetes 

 revolves around their role as causative agents 

 of disease, especially a disease in cattle 

 known as "actinomycosis" or "lumpy jaw." 



F(>rdinand Cohn's first description of an 

 actinomycete was based upon his study of an 



