504 



NITROBACTER 



to form nitrite within a range of pH 4:-8-7-3. Similar organisms have been described 

 by Cutler and Crump (1933), who found that no fewer than 104 out of 229 strains 

 of bacteria isolated from beet sugar effluent were able to produce nitrite from 

 ammonium salts. Fremlin (1903, 1914, 1929-30) has worked for a long time on 

 a very active nitroso-bacterium that grows in association with other organisms. 

 S. Winogradsky and H. Winogradsky (1933) have described two further genera of 

 nitrifying organisms — Nitrosocystis and Nitrosospira. Their article should be con- 

 sulted not only for an account of these organisms, but also for much useful informa- 

 tion on nitrifying bacteria in general. 



NITROBACTER 



Definition. — Nitrobacter. 



Cells rod-shaped, non-motile, not growing readily on organic media or in the 

 presence of ammonia. Cells capable of securing growth energy by the oxidation 

 of nitrites to nitrates. 



Type species. Nitrobacter winogradskyi. 



Nitrobacter winogradskyi was isolated by Winogradsky in 1891 — the year after 

 his discovery of N itrosomonas europaa. 



It is a small rod-shaped or pyriform organism, sometimes with one end drawn 

 out or recurved. Size 0-5 ju X 0-25-0-3 /n. Non-motile. Arranged in more or 

 less dense masses. 



Winogradsky (1896) cultivated it in a medium of the following composition : 



The medium is placed in a shallow layer in wide-bottomed flasks. Growth 

 occurs in the form of a just perceptible gelatinous film at the bottom of the flask ; 

 no turbidity appears. The nitrite is oxidized to nitrate. No organic nitrogen nor 

 carbon is required. Indeed the presence of organic matter hinders its develop- 

 ment, though not so markedly as that of the nitrite organism. On a washed- 

 agar medium made up with sodium nitrite, sodium carbonate, and potassium 

 phosphate (Omeliansky 1899) single colonies are formed, light brown in colour 

 and of irregular shape. Similar colonies, but smaller and more compact, appear 

 on silicic acid gel plates. No growth occurs in broth. 



In artificial culture the nitrate bacillus is very susceptible to the presence of 

 ammonia ; in soil it is less so. The accumulation of nitrate to the extent of 

 25 gm. per litre arrests the reaction (BouUanger and Massol 1903). 



As well as the genera that we have described, there are numerous other 

 organisms playing an important part in soil metabolism. We shall confine our- 

 selves here to giving a definition of the genera Hydrogenomonas, Methanomonas^ 

 Carboxydomonas, and Acetobacter. 



