CHAPTER XII 



Soil Actinomyces 



General considerations. The actinomyces, or ray fungi, are a group 

 of organisms present in great abundance in nature, particularly in the 

 soil. In the case of some soils, as many as 40 to 60 per cent of the col- 

 onies developing on the common agar or gelatin plate are actinomyces. 

 Their abundance and importance in the soil was rather overlooked 

 until the early part of this century (Hiltner and Stormer, Beijerinck), 

 while a systematic study of their occurrence and role in the soil ap- 

 peared only with the work of various investigators begun about 1913. 

 This was due primarily to the fact that these organisms grow only slowly 

 on the agar plate or on other common nutritive media. They form 

 colonies which are, at early periods of incubation, hardly distinguishable 

 from bacterial colonies. The fine mycelium, which breaks up readily into 

 a number of fragments during the process of staining, has also contrib- 

 uted to the failure of a proper study of the morphology of this group of 

 organisms and their classification. The actinomyces are characterized 

 by extreme uniformity of growth on complex organic media and 

 extreme variability of growth on synthetic media. On nutrient agar, 

 potato agar and other organic media, various organisms which are other- 

 wise distinctly different will look alike, as indicated by pigment produc- 

 tion, intensity of pigmentation, etc. When synthetic or other specific 

 media are used for purposes of differentiation, organisms closely related 

 show distinctive cultural differences. Early workers in soil bacteri- 

 ology grouped all the actinomyces into two or more large classes, charac- 

 terized by pigment formation (chromogenic species) or formation of 

 white aerial mycelium on nutrient agar. Hundreds of new species can 

 be described and differentiated by minor cultural differences, such as 

 intensity of pigmentation, rapidity of liquefaction of gelatin or starch, 

 formation of zones on specific media, etc. It is only very recently that 

 an attempt has been made to separate the actinomyces group into defi- 

 nite species and even genera, on the basis of morphological characters. 



General description of the genus Actinomyces. Actinomyces Harz 

 can be readily recognized and easily differentiated from the bacteria 

 and other fungi. The genus is characterized by the formation of a 



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