76 PRINCIPLES OF SOIL MICROBIOLOGY 



commonly found in single cells or in pairs. The thermal death-point is 

 56° to 58°C. The colonies on agar are rounded and light-brown, in 7 to 

 10 days at 28°C. After two weeks, they are darker in color; deep 

 colonies are 30 to 50 /j. in diameter and surface colonies are 50 to 150ju 

 in diameter, with a tendency to spread. The nitrate bacteria may also 

 be isolated directly from the silica gel plate, when nitrite is used as the 

 only source of energy. 



Beijerinck 33 claimed that the nitrate-forming bacterium can grow in 

 the presence of various organic substances but that the organism loses 

 some of its power of oxidation after growing in the presence of soluble 

 organic matter. Accordingly, he suggested that growth and oxidation 

 are two distinct functions and that even if small amounts of organic 

 substances (0.05 per cent) did not prevent the oxidation of nitrite to 

 nitrate, reproduction of the organism in solutions containing large 

 amounts of these substances caused a stable modification in their 

 physiology. 



However, Fred and Davenport 34 obtained no evidence to support the 

 statements of Beijerinck. Contrary to the general opinion, they found 

 that certain forms of organic matter benefit rather than injure the 

 nitrate-forming bacterium. They grew the Nitrobacter on washed 

 nitrite agar and on slants of Niihstoff-Heyden agar with or without 

 N0 2 -ion present. The organism does not reproduce in Nahrstoff- 

 Heyden solution, which is non-toxic, while beef-infusion and peptone- 

 beef infusion, in higher concentrations, are toxic. The harmful ma- 

 terial is non-volatile and can be removed by extraction with ether 

 or alcohol. Nitrobacter will live 2 to 6 weeks in 1 per cent solutions of 

 Nahrstoff-Heyden, gelatin, peptone, casein, yeast water and milk, 

 and also in distilled water without any further development. Gelatin, 

 peptone, casein, skimmed milk, beef infusion and beef extract do not 

 decrease the oxidation of nitrite by the organism; asparagine, 

 (NH 4 )oS0 4 , and urea retard the oxidation; Nahrstoff-Heyden increases 

 it above water controls; sealed agar slants of Nitrobacter were kept 

 more than a year without serious injury to their power of oxidation. 



Winogradsky 35 definitely pointed out that growth and nitrite oxida- 



33 Beijerinck, M. W. tlber das Nitratferment und iiber physiologische Art- 

 bildung. Folia Microb. 3: 91-113. 1914. 



34 Fred, E. B., and Davenport, A. The effect of organic nitrogenous com- 

 pounds on the nitrate-forming organism. Soil Sci. 11: 389-407. 1921. 



36 Winogradsky, S. Sur la pretendue transformation du ferment nitrique en 

 espece saprophyte. Compt. Rend. Acad. Sci. 175: 301-303. 1922. 



