CULTURAL CHARACTERISTICS 169 



number, although certain variations occur. Occasionally there is 

 no acid from dextrose. Some of the cultures reduce nitrates. The 

 power of producing fluorescence is often lost. These variations may 

 indicate the existence of separate species that are now grouped 

 under this one head." 



Actinomyces from 12 to 50 per cent, of the organisms found in 

 soil are the Actinomycetes. This genus, the Actinomyces Harz, em. 

 Gasperini, is characterized by the possession of a mycelium composed 

 of hyphse which show true branching, like those of the higher fungi 

 (seldom measuring over 2 microns in diameter), and judging by 

 their staining reactions resemble true bacteria in their protoplasmic 

 properties. Their growth is not wholly within the agar or gelatin 

 medium upon which they have been inoculated; for when condi- 

 tions favor, an aerial mycelium is produced. In the aerial mycelium 

 "conidia" are formed. These conidia are sometimes round, some- 

 times oval, and sometimes rod-shaped. They resemble bacteria 

 closely in size, shape, and staining properties. They are generally 

 between 0.6 and 1.5 microns in diameter, and if oval or rod-shaped, 

 between 1 and 2 microns long. They stain readily with ordinary 

 bacterial stains, and in a microscopic preparation which does not 

 contain any hyphse, often cannot be distinguished from true bac- 

 teria. In many cases deep-stained granules show at the poles, 

 strongly suggestive of the metachromatic granules of the diphtheria 

 organism. According to Sanfelice, some of the actinomycetes are 

 acid-fast like the tubercle organism. The diphtheria and tubercle 

 organisms, moreover, sometimes produce branching forms, and some 

 writers place these two organisms in the same group with Actino- 

 myces. 



The growth of actinomycetes on solid media is very characteristic. 

 The mass of growth is generally of a tough, leathery consistency, 

 sometimes smooth, sometimes wrinkled, and often piled high above 

 the surface of the medium. Often the mass is brilliantly col- 

 ored, and the color produced varies greatly with differences in the 

 composition of the medium, but with constant composition of the 

 medium, the color of the growth may be characteristic of the species. 

 The aerial mycelium, often produced above this growth, may also 

 be brilliantly colored and of an entirely different color from the mass 

 of growth beneath it. Sometimes the aerial hyphse are short and 

 give the growth a chalky or mildewy appearance; but often they are 

 long enough to cover the growth with a light, delicate nap, 1 or 2 mm. 

 thick. Some species produce pigments that diffuse through the 

 medium, coloring it gray, yellow, brown, red, blue, or green. The 

 color varies with the species and with the composition of the medium. 

 It is not so definitely characteristic of the species as is the color of 

 the growth itself or of the aerial mycelium; but with a medium of 

 constant composition, the color produced is of considerable value 



