OXIDATION OF SULFUR IN THE SOIL 243 



days old) cultures. The organism stains well with gentian- 

 violet and methylene blue. It is Gram-positive. 



CULTURAL CHARACTERISTICS 



No agar or other solid medium has been found as yet, upon 

 which the organism would grow. It grows in licjuid media with 

 a strong uniform clouding, without any surface growth or sedi- 

 ment formation. It does not grow on the common organic 

 media, although the presence of glucose or peptone in the medium 

 is not injurious. Inorganic media containing sulfur as a source 

 of energy are suited for its growth. In the presence of tri- 

 calcium phosphate, the growth of the organism is accompanied 

 by characteristic reactions: the sulfur forming originally a layer 

 on the surface of the medium usually drops to the bottom, the 

 sulfuric acid formed from the oxidation of the sulfur dissolves the 

 tri-calcium phosphate giving soluble phosphate and CaSOr 2H2O, 

 the calcium sulfate crj^stallizes out in the form of radiating mono- 

 clinic crystals hanging down from the sulfur particles that are 

 floating on the surface of the medium or protruding upward 

 from the bottom. The reaction of the medium becomes acid 

 as indicated by the change in the hydrogen-ion concentration. 

 At a pH of about 2.8, the reaction becomes stationary till all the 

 calcium-phosphate has been dissolved. In the presence of an 

 excess of this neutralizing agent, or in the presence of rapidly 

 dissolving alkaline carbonates, the culture is injuriously affected. 

 Anything that will tend to change the medium to an alkaline or 

 even a less acid reaction (except, of course, the action of the 

 buffers), such as shaking the culture, in the presence of even 

 smaller amounts of tri-calcium phosphate, will also tend to 

 affect the uniform growth of the organism injuriously. 



The culture can be kept alive for numerous consecutive genera- 

 tions on the liquid media and when not injured by an excess of 

 alkali or acid, may be as active as a recently isolated culture. 



The index No. of ThiobaciUus thiooxidans is, according to 

 the new Descriptive Chart of the Society of American Bacteriolo- 

 gists, 5332-5230-2222. 



