234 SELMAN A. WAKSMAN 



substances for their growth. Keil claims to have isolated pure 

 cultures of Beggiatoa and Thiothrix, and found these organisms 

 to be able to live in media free from any traces of organic matter, 

 although the presence of small quantities of organic substances is 

 not detrimental to these organisms. Ammonium salts are used 

 as sources of nitrogen and only carbonic acid as a source of carbon. 

 Carbon dioxide pressure may vary within the limits of 0.5 and 

 350 mm. (25 mm. is the optimum) ; oxygen may vary within 10 

 to 20 mm. and H2S within 0.6 to 1.7 mm. The presence of 

 carbonates is important for the neutralization of the acids. The 

 organism seems to assimilate carbon at the rate of 1 gram per 

 8 to 10 grams of sulfur oxidized (see also Hinze and INIolisch). 

 The Thioplaca has been studied in detail by Wisloch and 

 Kolkwitz. 



The second group of the sulfur oxidizing bacteria consists of 

 colorless organisms forming no threads and containing sulfur 

 within their cells. The following forms belong to this group: 

 Monas (Hinze (1913)), Thiophijsa (Hinze (1903)), Thiovulum 

 (Hinze (1913)), Spirillinn (Molisch (1912)), Thiospirillum (Ome- 

 lianski (1905)), Bacteman bodsta (Molisch (1912)), Bacillus 

 thiogenes (Molisch (1912)), and Achromatium (Nadson (1903), 

 and Griffith (1913)). 



Some idea of this group of organisms is obtained from a ref- 

 erence to the work of a few investigators. Jegunow (1896) 

 studied the oxidation of the hydrogen sulfide formed in the mud 

 of the Liman in Odessa and in the Black Sea. He described two 

 sulfur bacteria; Thiobacterium a, a motile, colorless, slightly 

 curved organism, 4.5 to 9^ long and 1.4 to 2.3^ wide, containing 

 a finely granulated plasma and large sulfur granules. Thio- 

 bacterium /3, motile, colorless, curved, 2.5 to 5 by 0.6 to 0.8//, and 

 containing a row of shining sulfur granules. Monas MiilleH was 

 described in detail by Hinze (1913), who has shown that this 

 organism belongs morphologically to the flagellates and physio- 

 logically to the sulfur bacteria. jNIost of these organisms were 

 isolated from the water. 



Gicklehorn (1920) recently described several new sulfur bac- 

 teria, of which tv;o are classified with this group, namely : Spiril- 



