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INTRODUCTORY 



Actinomycetes^ comprise a group of branching unicellular organisms, 

 hich reproduce either by fission or by means of special conidia. They 

 usually form a mycelium which may be of one kind— vegetative or sub- 

 strate—or of two kinds— vegetative and aerial. The actinomycetes are 

 related, on the one hand, to the true fungi or the Hyfhomycetes, with 

 which they have often been classified, and, on the other hand, to the 

 true bacteria or the Schizomycetes, with which they are usually included 

 for purposes of characterization and identification. In one of the early 

 definitions of the actinomycetes (321) they were described as "unicellu- 

 lar microorganisms, liJ, in diameter, filamentous, branching monopodi- 

 ally, seldom dichotomous, producing colonies of radiating structure. 

 They reproduce by fragmentation or oidia-formation; both kinds of 

 spores grow in ordinary media to form filamentous mycelium, never 

 growing into a rod-shaped vegetative state." 



Frequently, the actinomycetes have been looked upon as a separate 

 group of organisms occupying a position between the filamentous fungi 

 and the bacteria. It has even been suggested that the actinomycetes be 

 considered not only as forming the link between fungi and bacteria, but 

 as representing the original prototypes from which both of these groups 

 of organisms have been derived. Some of the actinomycetes are known 

 to have their counterparts among the bacteria, and others among the 

 fungi. The fact, however, that actinomyces mvcelium and spores are 

 similar in diameter to those of bacteria suggests the advisability of clas- 

 sifying the actinomycetes among the bacteria. A separate order has, 

 therefore, been created, the Actinomycetales, which is distinct from the 

 Euhacteriales, or the true bacteria. 



Actinomycetes are of universal occurrence in nature. They are 

 found in large numbers in soils, in fresh waters, in lake and river bot- 

 toms, in dust, on plant residues, on food products, in manures, and in 

 composts. They are known to cause various important plant and animal 

 diseases. Occasionally, they induce certain forms of food spoilage, es- 

 pecially because of the peculiar musty odor that they impart. 



Notwithstanding an extensive literature dealing with the actinomy- 

 cetes, many aspects of their nature and physiolog)^ and even of their role 

 in various natural processes, are still little understood. This is due to 



^ The word "actinomycetes" is used in this treatise to designate the organisms 

 under discussion in a plural sense; the words "actinomyces" and "actinomycete" 

 are used in a singular sense, without reference to any specific form, whether it 

 be a member of the genus Actinomyces, or that of any of the other three genera. 



