THE BIQL.OGICAL ACTIVITIES OF BACTERIA 57 



the plant as a parasite upon the bacteria, in that it derives the sole 

 benefit from the relationship and eventually bodUy consumes its host. 

 Nitrifying Bacteria. — A process diametrically opposed in its chem- 

 istry to denitrification and reduction is that which brings about an 

 oxidation of ammonia to nitrites and nitrates. The actual increase 

 of nitrates in soil allowed to stand for any length of time and examined 

 from time to time has been a well-established fact for many years; but 

 it was believed until a comparatively short time ago that this increase 

 was due to a simple chemical oxidation of ammonia by atmospheric oxy- 

 gen. The dependence of nitrification upon the presence of living organ- 

 isms was finally proved by Muntz and Schlossing ' in 1887, who demon- 

 strated that nitrification was abruptly stopped when the soil was 

 sterilized by heat or antiseptics. It remained, however, to isolate and 

 identify the organisms which brought about this ammonia oxidation. 

 This last step in our knowledge of nitrification was taken in 1890, by 

 Winogradsky. Winogradsky ^ found that the failures experienced by 

 others who had attempted to isolate nitrifying bacteria were due to the 

 fact that they had used the common culture media largely made up of 

 organic substances. By using culture media containing no organic 

 matter Winogradsky succeeded in isolating free from the soil, bacteria 

 which have since that time been confirmed as being the causative factors 

 in nitrification. During his first experiments this author observed that 

 in some of his cultures the oxidation of ammonia went only as far as the 

 stage of nitrite formation, while in others complete oxidation to nitrates 

 took place. Following the clewS indicated by this discrepancy, he 

 finally succeeded in demonstrating that nitrification is a double process 

 in which two entirely different varieties of microorganisms take part, 

 the one capable of oxidizing ammonia to nitrites, the other continuing 

 the process and converting the nitrites to nitrates. The nitrite-forming 

 bacteria discovered by Winogradsky, and named Nitromonas or Nitro- 

 somonas, are easily cultivated upon aqueous solutions containing am- 

 monia, potassium sulphate, and magnesium carbonate. According to 

 their discoverer they develop within a week in this medium as a gelat- 

 inous sediment. After further growth this sediment seems to break 

 up and the bacteria appear as oval bodies, which swim actively about 

 and develop flagella at one end. Upon the solid media in ordinary use 

 they can not be cultivated. Special solid media suitable for their cul- 



> Murdz und Schlossing, Compt. rend, de I'acad. des sciences, 1887. 

 2 Winogradsky, Ann. Past. Inst., iv and v, 1890, 1891. 



