238 SELMAN A. WAKSMAN 



To summarize: (1) The microorganisms concerned in the 

 sulfur cycle are separated into reducing bacteria and oxidizing 

 bacteria, the latter being the true sulfur bacteria. (2) The true 

 sulfur bacteria are divided into five groups: the first three 

 groups of sulfur bacteria are found in sulfur springs, canal and 

 mud waters, curative muds, river water and sea water; they oxi- 

 dize hydrogen sulfide and sulfides, but not elementary sulfur, 

 and accumulate sulfur within their cells; the fourth group of 

 bacteria, consisting of small rod shaped organisms, is found in 

 sea water, canal water and soil; these bacteria are able to oxi- 

 dize hydrogen sulfide and other sulfides, thiosulfates and ele- 

 mentary sulfur, forming a heavy pelhcle on the surface of the me- 

 dium and allowing an accumulation of sulfur outside of their cells; 

 the fifth group of sulfur bacteria occurs in soils to which ele- 

 mentary sulfur has been added, particularly in soil-sulfur-com- 

 posts, oxidizing primarily elementary sulfur, thiosulfates to a 

 small extent, but not hydrogen sulfide or sulfides; these bacteria 

 grow uniformly throughout the medium, not forming any peUicle, 

 do not liberate any sulfur and allow a very intensive production 

 of sulfuric acid, and the necessary carbon is derived entirely 

 from the carbon dioxide of the atmosphere; the fifth group is 

 morphologically related to group four, but includes organisms 

 very small in size and the strongest sulfur oxidizing and acid pro- 

 ducing bacteria known. 



