72 



Occurrence of Microorganisms in Soil 



energy obtained by oxidation of hydrogen gas has been utihzed for 

 reduction of nitrates by Hydro genomonas agilis. 



Decomposition of cellulose in the soil may be due to the symbiotic 

 action of two bacteria, one reducing nitrate to atmospheric nitrogen 

 and the other decomposing cellulose; the products of the cellulose- 

 decomposing bacteria are used by the other organism as sources of 

 energy, thus enabling it to reduce the nitrate, whereas the oxygen 

 thus liberated makes it possible for the cellulose-decomposing or- 

 ganism to live under anaerobic conditions. 

 Sulfate- Reducing Bacteria. Several 

 organisms capable of reducing sulfate to 

 hydrogen sulfide have been described. 

 Vibrio desulfuricans was isolated from 

 soil and other substrates. It is a strictly 

 anaerobic, Gram-negative form, growing 

 at 30-55 °C, and able to use salts of or- 

 ganic acids as sources of energy. The 

 cultures grown at the lower temperature 

 (30°) consist of small vibrios or spirals, 

 motile by means of one or two polar 

 flagella, and asporogenous; at higher tem- 

 perature (55°C), the cells are largely 

 vibrios, containing granules. Starkey 

 found that they form spores and sug- 

 gested that the name be changed to 

 Sporovibrio desulfuricans. 

 Urea-Decomposing Bacteria. Pasteur was the first to recognize, 

 in 1860, that ammonia formation from urea is brought about by a 

 living organism, which he designated as Torula ammoniacale. It was 

 later established that organisms capable of decomposing urea are 

 found in most families of bacteria, actinomycetes, and fungi, but that 

 only certain specific bacteria, whose metabolism is closely connected 

 with the transformation of this substance, are termed "urea bacteria." 

 These are divided into cocci and bacilli; the former are usually de- 

 stroyed at 60-70°, whereas the latter, because of their ability to form 

 endospores, can withstand heating at 90-95° for several hours. The 

 optimum temperature is about 30°C. These organisms usually thrive 

 best in media containing urea (2 per cent), particularly when made 

 alkaline with ammonium carbonate. The accumulation of ammonia 

 from the hydrolysis of the urea in the culture is so great as to kill, in 

 many instances, the organisms themselves. Rapid urea decomposi- 



FiG. 31. Sulfate-reducing 

 bacterium, Spirillum desul- 

 furicans ( from Beijerinck 

 and Omeliansky ) . 



