Aiitolropliic Ikicteria 67 



pluitc is tianslonncd into soluble loims by the acid protluccd from 

 the sulfur. A direct correlation has been found between the acid 

 formed and the amount of phosphate going into solution. When a 

 fresh mixture is inoculated with some material from an old compost, 

 the reaction goes on more rapidly, indicating the biological nature 

 of the process. 



15y inoculating, with some of the above compost, a medium con- 

 taining sulfur as the only source of energy, certain mineral salts, and 

 tricalcium phosphate as a neutralizing agent, the culture of a bac- 

 terium capable of oxidizing sulfur to sulfuric acid is obtained. The 

 acid produced interacts with the tricalcium phosphate and trans- 

 forms it into calcium sulfate and monocalcium phosphate and finally 

 into phosphoric acid. 



By use of very acid media, with an initial reaction of pH 2.0 and 

 a high dilution of the crude culture (1:100,000), a pure culture of 

 an organism was obtained from such composts. This culture gave 

 no growth when inoculated into broth or other mecha favorable for 

 the growth of bacteria and fungi. Microscopic examinations further 

 established the purity of the organism described as Tli. thiooxidans. 

 It is a small, nonmotile rod, 0.75-1.0 by 0.5-0.75 /i; it produces cloudi- 

 ness tliroughout the medium but does not form any pellicle. 



The organism is strictly aerobic, the particles of sulfur in the cul- 

 ture being surrounded by the bacterial cells. The medium becomes 

 very acid. In the presence of calcium phosphate or carbonate, the 

 sulfuric acid produced in the medium interacts with the calcium 

 to gi\ e crystals of CaS04 • 2H2O, which hang down from the particles 

 of sulfur floating on the surface of the medium; gradually they fill 

 the flask with gypsum crystals. 



The organism forms no spores and is destroyed at 55-60 "C in sev- 

 eral minutes. The limiting reactions are pH 6.0 and 1.0. It is pos- 

 sible, howe\"er, to accustom the organism to a neutral and even an 

 alkaline reaction, when transferred from one soil to another before 

 the reaction becomes too acid. 



Most of the other sulfur bacteria, especially the filamentous forms 

 {Beggiatoa, TJiiotlirix), occur largely in water basins. 



Van Niel has shown that the metabolism of purple and green 

 sulfur bacteria (Thiorhodaceae) is a truly photosynthetic process 

 of the general reaction: 



CO2 + 2H2A = CH2O + H2O + ^2A 

 The green bacteria dehydrogenate the H2S only to S, whereas the 



