610 SELMAN A. WAKSMAN 



cc. of distilled water. This medium is sterilized in the autoclave, 

 at 15 pounds for fifteen minutes. 



When a small particle of soil is added to media 6 and 9, 

 growth may or may not take place according to the nature 

 of the soil, the two media behaving in a distinctly difYerent 

 way. Medium 6 is acid in reaction and contains elementary 

 sulfur as the only source of energy; only those organisms 

 which can oxidize elementary sulfur and can live at a pH of 4.0 

 and less will develop in this medium. Such organisms are not 

 usually present in common cultivated soils, unless the soils have 

 had previous applications of elementary sulfur. The organism 

 developing on this medium is the Th. thiooxidans Waksman and 

 Joffe (1922) described in detail elsewhere. This organism is not 

 universally present in the soil, but is present in soil mixed with 

 powdered sulfur, as around the sulfur mines, etc. This is the 

 reason why the organism was originally isolated from composts 

 of sulfur with soil or sulfur, rock phosphate and soil. This 

 organism oxidizes sulfur, in pure culture, quantitatively to sul- 

 furic acid; it also oxidizes thiosulfate, without the separation of 

 elementary sulfur, with active acid formation. The growth of 

 this organism on the liquid media is quite characteristic: it 

 forms, in four to five days, at 25°C., a turbidity throughout the 

 medium, without any pellicle formation. In the presence of 

 calcium salts, crystals of gj-psum soon appear, both on the bottom 

 of the flask and hanging down from the particles of sulfur. The 

 sulfur floating on the surface of the medium drops to the bottom. 

 When transferred to medium 10, minute pale to pale-yellow to 

 dirty-yellow (or light cream colored) colonies appear in five to 

 six days. In the presence of insoluble calcium salts, like the 

 carbonate and the phosphate, a colorless halo is rapidly formed 

 around each colony on the plate, due to the action of the sulfuric 

 acid formed from the oxidation of the thiosulfate upon the in- 

 soluble calcium salts. A ring of gypsum crystals is formed 

 around each colony. 



Medium 8 behaves differently; organisms that can thrive at 

 distinctly alkaUne reactions and that can oxidize thiosulfate will 

 develop on this medium. Such organisms are universally dis- 



