2 



Scientific Proceedings (109). 



the distinguishing differences being the facts that they oxidize 

 H 2 S, accumulate sulfur within their cells, are colorless and non- 

 thread forming. Some of these have been isolated in pure culture. 

 (3) Purple bacteria. Some of these seem to play a part in the 

 sulfur cycle, although none of the sulfur forms have yet been 

 isolated in pure culture. (4) Colorless, non-thread forming 

 sulfur oxidizing bacteria which do not accumulate sulfur within 

 their cells, but which produce an abundance of sulfur (from H 2 S 

 and thiosulfates) outside of their cells. The two characteristic 

 and most important forms belonging to this group are the Thio- 

 bacillus denitrificans , anaerobic, deriving its oxygen from the 

 decomposition of nitrates; and Thiobacillus thioparus, which 

 oxidizes thiosulfates, H 2 S, and S, allows an extensive accumulation 

 of sulfur from the first two and allows the medium to become 

 distinctly acid. The work on this group of organisms has been 

 carried on chiefly by Beijerinck and associates. 



It is suggested here to add another group of sulfur-oxidizing 

 bacteria, which was isolated and is being extensively studied at 

 the New Jersey Agricultural Experiment Station. This group 

 (5) will comprise bacteria similar to group 4 in their morphology, 

 although much smaller in size (less than 1 /jl. in length) and dis- 

 tinctly different physiologically. They do not act upon thiosul- 

 fates and H 2 S, only upon elementary sulfur and allow the medium 

 to become acid up to a P H of 0.8-1.2. 



These organisms have been isolated from composts consisting 

 of sulfur, phosphate rock and soil, where the sulfur oxidation is 

 very strong; they were isolated by means of a purely inorganic 

 culture medium, consisting of minerals with elementary sulfur 

 as the only source of energy. These organisms are autotrophic 

 and do not need any organic substances for their development, 

 the carbon being derived from the C0 2 of the air. The sulfur is 

 oxidized very rapidly with the production of sulfuric acid. When 

 the medium is poor in neutralizing substances as well as in inor- 

 ganic buffering substances the accumulation of acids will soon 

 reach such a concentration that the growth of the organisms may 

 be stopped; in the presence of the proper neutralizing agents, 

 such as tricalcium phosphate, sodium bicarbonate, or in the pres- 

 ence of sufficient amount of buffering agents, such as di-basic- 



