BACTERIA REDUCING NITRATES AND SULFATES 189 



The medium is filled to the neck of the bottles, then inoculated 

 and incubated at 25°C. Sulfur reduction becomes evident by a 

 change in color due to formation of H 2 S. The bacterium can be 

 isolated from the soil when some sodium sulfite is added to the medium. 

 The presence of organic substances as sources of energy and anaerobic 

 conditions are required for the action of the organism. For the isolation 

 of pure cultures, 10 per cent gelatin or 2 per cent agar is added to the 

 above medium; in place of FeS0 4 a trace of FeSO^NH^oSO^GIi^O 

 together with some sodium carbonate is used. In 3 to 6 days small 

 black colonies appear. Sulfur is deposited in the colony, on the solid 

 medium, among the bacterial cells, due to the incomplete reduction of 

 the sulfate. The organism is a very motile spirillum, 4 by lju in 

 size and is strictly anaerobic. 



Another organism {Microspira aestuarii) was isolated from sea water. 

 Various thermophilic bacteria (Vibrio thermo desulfuricans) are capable 

 of reducing sulfates. 04 These three forms are closely related to one 

 another and have the ability, apart from all other bacteria, to utilize 

 sulfates and thiosulfates as sources of oxygen under anaerobic conditions. 



An actinomyces (A. pelogenes) capable of reducing sulfates to sulfides 

 and forming iron sulfide was also isolated. 65 These organisms occur in 

 great abundance in certain lakes and seas, and especially in the black 

 curative muds; their reducing properties under these conditions keep 

 the sulfur in the process of constant transformation, 66 as discussed in 

 detail elsewhere (p. 611). 



64 Elion, L. A thermophilic sulfur-reducing bacterium. Centrbl. Bakt. II, 

 63: 58-67. 1924. 



65 Sawyalow, W. tJber Schwefelwasserstoffgiirung im schwarzen Heilsch- 

 lamme. Centrbl. Bakt. II, 39: 440-447. 1913. 



66 Nadson, G. A. On the hydrogen sulfide fermentation in the Weissovo Lake 

 and the participation of microbes in the formation of the black mud. 1903. 

 St. Petersburg. (Russian.) 



