Waksman — 10 — Actinomycetes 



As early as 1894, a number of species were already recognized. The 

 names of many of them were considered as synonymous, as shown in 

 Table 1. 



Systematic Position and Classification of Actinomycetes:— 



Relation of actinomycetes to bacteria and fungi.— Although they 

 are very often grouped with the fungi, the actinomycetes are related in 

 many respects to the bacteria and are usually classified with the bacteria 

 under the Schizomycetes. 



The relationship of the actinomycetes to the bacteria is based upon 

 the following properties: 



1. The diameters of the filaments and spores of actinomycetes are similar to 

 those of true bacteria and not of fungi. 



2. Many actinomycetes reproduce by fragments or oidia that are similar in 

 size and in shape to the rod-shaped and spherical bacteria. 



3. Many actinomycetes, especially the pathogens, produce no aerial mycelium; 

 their growth appears similar to that of pleomorphic bacteria, like the members of 

 the genus Corynebacterium. 



4. Many actinomycetes are acid-fast, and in their morphology and physiology 

 resemble true bacteria, namely, the members of the genus Mycobacterium. Cer- 

 tain groups among the actinomycetes, especially the genera Actinomyces and 

 Nocardia, show a close resemblance to the mycobacteria. 



That the actinomycetes show a definite relationship to the fungi, 

 especially the Fungi Imperfecti, is brought out by the following proper- 



1. The manner of braliching of the aerial mycelium of many representative 

 groups of actinomycetes, especially the genera Streptomyces and Micramonospora, 

 definitely resembles that of fungi. 



2. The production by a large number of actinomycetes of an aerial mycelium 

 and of conidia is definitely typical of many true fungi. 



3. The growth of the colonies on the surfaces of liquid and of solid media, as 

 well as their growth in a suspended or submerged condition, is similar to that of 

 fungi and not of true bacteria. Turbidity is not usually produced in the liquid 

 culture. 



One may, therefore, conclude that the actinomycetes comprise many 

 highly heterogeneous groups of organisms, varying greatly in their mor- 

 phological characteristics, and resembling in some respects true bacteria 

 and in others true fungi. For these reasons, the actinomycetes may ten- 

 tatively be placed in a taxonomical transition group between the Schizo- 

 mycetes and Hyphomycetes, with considerable similarity to, if not actual 

 overlapping of, one or the other. 



Classification of actinomycetes.— l!^?Lny systems of classifying the 

 actinomycetes have been suggested. These are based upon their activi- 

 ties in a natural environment such as pathogenic and nonpathogenic 

 forms, upon their cultural characteristics such as pigmentation and gela- 



