Isolation of Nitrogen Bacteria 



171 



The first reaction involves the change of ammonia 

 to nitrite, the second change of a nitrite to a nitrate. 



The Russian bacteriologist, Winogradsky, not only 

 confirmed Warrington's observations, but, by a series 

 of highly ingenious experiments, made clear the causes 

 of the failure of other bacteriologists. He showed, in 

 1890 and 1891, 

 that the nitrify- 

 ing-organisms do 

 not develop in the 

 culture solutions 

 and on the gela- 

 tine plates em- 

 ployed for the 

 isolation of vari- 

 ous soil bacteria. 

 He was, therefore, 

 led to employ cul- 

 ture media con- 

 taining only min- 

 eral salts. The gelatin was replaced by the mineral 

 silica jelly, on which colonies of the nitrifying bacteria 

 developed without great difficulty. 



The organisms capable of changing ammonia to a 

 nitrite were found by him in two varieties, one from 

 the Old World, which he named Nitrosomonas, the 

 other from the New World and named Nitrosococcus. 

 Both of these belong to the group of nitrous ferments 

 and are spherical in shape. The so-called nitric ferments, 

 those capable of changing nitrites to nitrates, were 

 designated by him as Nitrobacter. The latter are small, 



Fig. 27. Nitrite bacteria, showing the spherical 

 form of individual cells. 



