72 PRINCIPLES OF SOIL MICROBIOLOGY 



in the form of a zooglea-like zone around the soil particles, and can be readily 

 isolated by transferring to sterile liquid medium. 



Morphology of the nitrite forming bacterium. In the process of nitrifica- 

 tion we are dealing not with one organism, but with a group of closely 

 related organisms. One strain was isolated 30 from soils of Western 

 Europe and was called Nitrosomonas (Nitr. europca Winogradsky) . 

 Another strain was isolated from soils of South America (Campinas- 

 Brazil, Quito-Ecuador) and Australia (Melbourne) and was called 

 Nitrosococcus. 



When a vigorous culture of Nitrosomonas is inoculated into the sterile 

 liquid medium, an appreciable nitrite reaction is obtained in 2 to 3 

 days, reaching a maximum in 5 to 6 days. When the culture is 

 examined microscopically, very few organisms are found in the super- 

 natant liquid, but rare, compact, variable (10 to 50/x) zooglea recognized 

 with difficulty are formed in the sediment. When a drop of KI-I 

 solution is added, the cells are easily recognized. In 8 to 10 days, the 

 liquid becomes opalescent and, on examination in a hanging drop, it 

 is found to consist of swarming, ellipsoidal, motile microbes, as seen in 

 Plate III. This shows the zooglea broken up into a swarm stage. 



The cells of the Nitr. europea are always oblong, similar to a zero, 

 never coccus-like, 1.2 to 1.8/j. long by 0.9 to 1/j, wide. They can be 

 stained with all ordinary basic anilin dyes and are Gram positive. 

 The motile cells of the swarm carry on one end a moderately long 



PLATE III 

 Nitrifying Bacteria 



5. Surface colonies of Nitrosomonas on silicic acid gel; stained with carbol 

 fuchsin, X 130 (from Gibbs). 



6. Surface colony of Nitrosomonas on silicic acid gel; stained with carbol 

 fuchsin, X 800 (from Gibbs). 



7. Nitrosomonas europea, X 660 (from Winogradsky). 



8. Nitrosomonas javanensis, X 660 (from Winogradsky). 



9. Colonies of Nitrobacter; deep-seated colonies on washed agar, unstained, 

 X240 (from Gibbs). 



10. Nitrobacter from culture in liquid medium; stained with carbol fuchsin, 

 X 1660 (from Gibbs). 



11. Nitrobacter from nitrite-agar cultures, 2 months old (from Fred and Daven- 

 port). 



12. Nitrobacter from nitrite-agar cultures, 15 days old, showing polar flagella 

 (from Fred and Davenport). 



30 Winogradsky, 1892 (p. 62). 



