NITROSOMONAS 503 



of ellipsoidal cells, intermediate between cocci and bacilli ; its dimensions are 

 1-1-1-8 ju X 0-9-1 -0 fz. It is motile by a single long flagellum. It is arranged 

 singly, or aggregated into a zoogloeal mass by some slightly viscous substance ; 

 chains even of 3 or 4 members are rare. 



Winogradsky found that this organism could grow in a medium devoid of all 

 traces of organic matter. It must therefore obtain its carbon from the magnesium 

 carbonate added to the medium ; that is to say, it can assimilate the carbon of 

 carbonic acid. It obtains its nitrogen from the ammonium sulphate, and oxi- 

 dizes it to nitrite. From purely inorganic substances it can therefore synthesize 

 organic matter — a process rarely accomplished independently of chlorophyll and 

 sunlight. 



Experiments showed that the amount of ammonia oxidized and the amount 

 of carbon assimilated ran strictly parallel. For every 96 mgm. of nitrous acid 

 formed it assimilated only 1 mgm. of carbon. This disproportion between the 

 rapid oxidizing and the slow assimilating action of the organism explains why its 

 growth is so slow. 



Winogradsky and Omeliansky (1899) showed that Nitrosomonas europaa was 

 very susceptible to the presence in the medium of organic nitrogenous substances 

 such as peptone or asparagin. In fact the more nutritious a medium was for 

 ordinary bacteria, the less suitable was it for the nitrite organism. It is possible, 

 however, for it to grow in the presence of organic matter (Boullanger and Massol 

 1904, Fremlin 1914, Bonazzi 1919), but in artificial culture the results are not 

 satisfactory. On the ordinary laboratory media, for example, growth is scanty 

 or entirely absent. In the soil it is probable that its susceptibility to the presence 

 of organic matter is less. 



For the study of single colonies the best medium is a silicic acid gel poured 

 into plates (Winogradsky 1891). After 3 or 4 weeks' incubation small, compact, 

 sharply-defined colonies appear, of a brownish colour. Growth can be hastened 

 by pouring a solution of ammonia over the plate. 



Nitrosomonas europrea is most active at a temperature of 25-30° C, in a well- 

 aerated medium contained in large flat-bottomed flasks, which are slowly agitated. 

 The presence of scoria (cellular lava) in the medium is beneficial, apparently by 

 increasing the surface exposed to the air (Boullanger and Massol 1903). Under 

 such conditions in an inorganic solution it may oxidize as much as 169 mgm. of 

 ammonia nitrogen to nitrite per litre of medium in 14 days (Bonazzi 1919). The 

 accumulation of nitrite arrests the reaction. The type species is known as Nitroso- 

 monas europaa, Winogradsky. Another similar organism, but of spherical shape, 

 is called Nitrosococcus americanus ; it is found in the New World. 



Nitrosomonas is not by any means the only organism capable of forming nitrite 

 from ammonia. Cutler and Mukerji (1931) isolated a number of organisms from 

 soil that were able to perform this oxidation, though not so actively as Nitrosomonas. 

 A full description of these organisms has not yet been published, but most of them 

 were Gram-positive, non-sporing, non-motile rods, 1-4— 1-9 jli long and 0-70-0-85 /li 

 broad, which grew on ordinary agar, which oxidized various ammonium salts to 

 nitrite, both in culture medium and in soil, which were unable to oxidize nitrite 

 to nitrate, and which failed to grow in the absence of oxygen. Some strains liquefied 

 gelatin and some did not. Nitrite formation was stimulated by the presence of 0-1 

 per cent, sucrose, but no growth occurred in pure sugar solutions. Unlike Nitro- 

 somonas, which requires a distinctly alkaline medium, these organisms were able 



