SOIL ACTINOMYCES 287 



than the mycelial filaments. They resemble bacteria in size, shape and 

 staining properties, are 0.5 to 1.5 microns in diameter, 1 to 2 microns 

 long, oval to rod-shaped. All actinomyces, particularly in young 

 preparations, are gram-positive. In liquid media, they never cause a 

 turbidity, but grow either on the surface of the medium or in the form 

 of flakes or small colonies throughout the medium; they may sink, espe- 

 cially the flakes, to the bottom of the container or adhere to the glass of 

 the tube. The surface colonies may grow together to form a smooth 

 or wrinkled membrane. The colonies on solid media are tough, 

 leathery, smooth or wrinkled, often growing high above the surface of 

 the medium, and are broken up only when appreciable effort is applied. 

 When transferred to suitable media, almost all the spores germinate, 

 while only a small part of the fragments of mycelium do so. The older 

 the mycelium, the more reduced is the germinating power of the indi- 

 vidual fragments. 1 



The aerial mycelium may be white, grey, red, yellow, brown, etc. 

 The aerial hyphae may be short, giving the growth a chalky appearance, 

 long and forming a thick mat over the surface of the growth, or in the 

 form of a fine network. The colonies are often brilliantly colored; 

 some species produce soluble pigments which vary in color and in- 

 tensity in accordance with the effect of the composition of the medium. 

 Most species are characterized by the production of a sharp peculiar 

 odor characteristic of the soil (earthy odor). All soil species liquefy 

 gelatin; the rapidity of liquefaction depends upon the nature of the 

 organism and previous cultivation. Most of them produce active dias- 

 tatic enzymes; fewer produce invertase; still fewer produce tyrosinase 

 which enables them to convert the tyrosin of the protein molecule into 

 dark colored melanins. 



Terminology and systematic position. More names have been applied 

 to this genus than to any other group of microorganisms. Among these, 

 in addition to the proper designation " Actinomyces," the terms Lepto- 

 thrix, Cladothrix, Oospora, Oidium, Discomyces, Nocardia, Streptothrix 

 and Microsiphonales have been used. The terms Leptothrix and 

 Cladothrix designate two groups of higher bacteria, having nothing in 

 common with the Actinomyces. Leptothrix consists of chains of rod- 

 shaped bacteria surrounded by a thick gelatinous sheath, which gives 

 them the appearance of threads, not forming any true or false branching. 

 Cladothrix is similar to Leptothrix, forming somewhat thicker threads 



1 Orskov, J. Investigations into the morphology of the 

 and Munksgaard. Copenhagen. 1923. 



