64 PRINCIPLES OF SOIL MICROBIOLOGY 



number of organisms, some even pathogenic in nature, are able to pro- 

 duce nitrates, they were valueless due to improper interpretation. The 

 traces of nitrates probably came from the atmosphere and the nitrites 

 from the reduction of the nitrates present in the medium. 9 The 

 French 4 - 5 and English 6 - 10 investigators were primarily chemists, while 

 the German bacteriologists 8 were so much under the influence of the 

 gelatin plate method of R. Koch that the absence of growth on that 

 medium was thought to indicate the entire absence of an organism. 



Winogradsky started out with the idea that we were dealing here with 

 an organism of unknown properties which does not develop on the 

 gelatin plate. Fresh from his work on the sulfur and iron bacteria 

 (1885-1888), whereby he recognized organisms which can derive their 

 energy from inorganic compounds, he reasoned that a source of energy 

 so abundant as ammonia would be likely to be utilized by microorgan- 

 isms. If so, the organisms concerned might show properties similar to 

 those of organisms oxidizing other inorganic substances. The principle of 

 elective culture was adopted, whereby conditions are made unfavorable 

 for the development of any other organism, except those that are able 

 to oxidize ammonium compounds. Winogradsky used a medium of the 

 following compositions: 



Ammonium sulfate 1 gram 



Potassium phosphate. 1 gram 



Tap water 1000 cc. 



Portions of this medium (100 cc.) were placed in flasks each of which 

 contained 0.5 to 1 gram basic magnesium carbonate. The flasks were 

 inoculated with a little soil and, when active nitrification took place, 

 transfers were made to fresh quantities of medium. The ammonia 

 disappeared in two weeks, while the di-phenylamine reaction for nitrates 

 appeared in four days and reached its maximum in another three to 

 four days. Gelatin plates made from the flasks gave various species 

 of bacteria and yeasts, but none of them was able to nitrify. This 

 confirmed the idea of Winogradsky that the organism in question 

 cannot develop on gelatin. After several transfers, the observation 

 was made that a bacterium was present on the magnesium carbonate 

 sediment, covering it in the form of a zooglea. Under the microscope, 



9 Winogradsky, S. Die Nitrifikation. Lafar's Handb. Tech. Myk. 3: 132 

 181. 1904. 



10 Frankland, G. C, and P. F. The nitrifying process and its specific ferment. 

 Trans. Roy. Soc. London, B, 181: 107-128. 1890. 



