1 9 1 9l BOXAZZI—NI TRIFICA TION 1 9 7 



showing within the limits of individual variations in the same 

 species the same physiological and morphological characters. 

 Although theoretically a ''pure culture'' is that culture originated 

 monocytogenetically, in practice it is seldom obtained. True pure 

 cultures are only possible when the development of one single 

 cell into a colony is controlled directly by microscopical observa- 

 tion; in ordinary technic this is not done. Although these 

 requisites are necessarily of the same importance when '-pure 

 cultures" of nitrite- and nitrate-producers are desired, the term 

 "pure culture" of nitrifying bacteria does not convey this same 

 criterion of "absolute purity." 



The simple fact that Winogradsky found the nitrate- and 

 nitrite-formers of different lands to possess in common the char- 

 acter of not growing in bouillon, in spite of their morphological 

 differences, and that he made of this a criterion of purity, is enough 

 to show the misleading interpretation given to the term "pure 

 culture." The criterion of purity formulated by Winogradsky 

 (12) is as follows: "Introduce a loopfuU of a nitrified culture in 

 ordinary bouillon and keep at 30° C. during 10 days; at the end 

 of this time the bouillon must not show turbidity. The purity 

 of the nitrifying organisms is then proven." 



That this criterion of purity is rather indefinite is shown by the 

 following statement from Jordan and Richards, who, discussing 

 the purity of some of their cultures, state that "we are not even 

 prepared to say that there may not have been a mixture of two or 

 more species in our flasks, all agreeing closely in morphological 

 characters, and in giving no growth on gelatin, but differing in 

 important physiological respects." Omelianski (6) states that 

 "after renewed ammonia additions it is necessary to control the 

 purity by microscopical examinations as well as reinoculation in 

 bouillon." 



Since Wimmer (9) found a culture of nitrifying organisms to 

 prove pure on bouillon of one reaction, while this same culture 

 proved impure when tested on bouillon of a more alkaline reaction, 

 the author has adopted in his work w^ith nitrite-formers the fol- 

 lowing cardinal points which form the basis for his criterion of 

 the purity of a culture: (i) the culture must be in full nitrification 



