SOIL ACTINOMYCES 303 



cooled and inoculated with the organism in question. After the organ- 

 ism has developed for a few days, the plates are covered with a dilute 

 solution of potassium iodide, to which some hydrochloric acid is added. 

 The plates, in which the nitrate is reduced to nitrite, are colored blue 

 since the nitrite liberates the iodine from the potassium iodide. 27 This 

 method, however, is not so reliable as the common method (a naphthala- 

 mine + sulfonilic acid) of nitrite estimation, since the starch may be 

 decomposed by the diastatic enzymes. 



Selenium and tellurium salts (0.01 per cent concentration) are reduced 

 by numerous actinomyces to elementary selenium and tellurium; the 

 colonies are colored deep red and deep black respectively, due to the 

 fact that the metals are deposited within the mycelium. The phenome- 

 non is intracellular. According to Husz, 52 various actinomyces isolated 

 from the soil can reduce organic arsenic compounds, similar to Pen. 

 brevicaule, with the production of the characteristic garlic odor. These 

 results could not be confirmed by Lieske. This may be due either to 

 the difference in the organisms employed or difference in methods. 



Production of odor. Most actinomyces grown on organic or synthetic 

 media produce a characterictic odor of freshly plowed soil. This is 

 particularly true of the organisms possessing a mildewy aerial mycelium. 

 The odor varies from earthy to musty. It was thought at first 53 that 

 this odor was formed by a specific organism. It was soon found that 

 this is the property of a large number of species and the odor production 

 is in itself a variable factor. The non-spore-forming organisms usually 

 do not produce any odor. The intensity of the odor depends on the 

 composition of the medium. Carbohydrates, particularly glycerol, 

 stimulate the odor formation. The odoriferous substance has not been 

 isolated yet. 



A few actinomyces, particularly thermophilic forms, produce a pleas- 

 ant fruity odor. 



Pigment formation. The property of pigment formation is wide- 

 spread amcng the actinomyces. No culture can be considered non- 

 chromogenic until it has been studied on protein media and a variety of 

 protein-free media. Three different kinds of pigment should be con- 

 sidered: that of the vegetative mycelium, of the aerial mycelium, and 



52 Husz, H. Zur Kenntnis der biologischen Zersetzung von Arsenverbin- 

 dungen. Ztschr. Hyg., 76: 361. 1914. 



63 Rullmann. Chemisch-bakteriologische Untersuchungen von Zwischen- 

 decken-fiihlungen mit besonderer Beriicksichtigung von Cladothrix odorifera 

 Diss. Miinchen. 1895. 



