232 SELMAN A. WAKSMAN 



their cells. (4) Bacteria oxidizing sulfur and sulfur compounds, 

 but accumulating sulfur outside their cells. (5) Bacteria oxidiz- 

 ing elementary sulfur and not accumulating any sulfur within or 

 without their cells. The first four groups are mentioned in the 

 latest texts and re\dews on sulfur bacteria, while the fifth group 

 has been suggested on the basis of the results presented below. 



It is of interest to note that in most of the work on sulfur 

 oxidation by bacteria, with the exception of that of Jacobsen 

 (1912),^ the starting point was not suKur itself, but hydrogen 

 sulfide, sulfides, or thiosulfates. The activity of the organisms 

 was judged not by the oxidation of suKur, as measured by the 

 production of sulfuric acid and sulfates and subsequent change 

 in reaction, but either by the disappearance of the sulfide in the 

 medium or by the appearance or disappearance of the suKur 

 granules within or without the microbial cell. Of importance is 

 also the fact that all the earher work and most of the later work 

 has been done with organisms present in canal water, mud water, 

 curative muds, and sea water and very Uttle attention was paid 

 to the microorganisms concerned in the oxidation of sulfur in the 

 soil. This is the reason for using hydrogen sulfide and sulfides 

 as a source of sulfur, since the latter substance, and not elemen- 

 tary suKur, is present or is produced abundantly under those 

 conditions. 



A detailed review of the oxidation of sulfur and sulfur com- 

 pounds by microorganisms is given in the papers of Omeliansky 

 (1904), Diiggeh (1919) and in the book by Kruse (1910). A 

 brief historical re\'iew is presented here for a better understanding 

 of the work that is to follow. 



The first group of suKur oxidizing bacteria (colorless, thread 

 forming) consists of three genera: Beggiatoa, motile, forming 

 no sheaths; Thiothrix, fastened, forming no sheaths, and Thio- 

 ploca, threadforming bacteria, surrounded with a jelly-like 

 sheath. The Beggiatoas were the first organisms to attract 

 attention as having to do with the oxidation of sulfur or its 

 derivatives. Cramer (1870) pointed out that the granules found 



'Bibliography is found at the end of the second article, in this series. 



