182 PRINCIPLES OF SOIL MICROBIOLOGY 



nitrite formers. Out of 109 species of bacteria tested by Maassen, 15 

 in a solution containing 5 per cent peptone and 0.5 per cent sodium ni- 

 trate, 85 were found capable of reducing nitrates to nitrites, especially 

 Bad. pyocyaneum; 46 reduced the nitrite to ammonia and 4 liberated 

 atmospheric nitrogen. Out of 28 species of bacteria studied by Klae- 

 ser, 12 all but one were found capable of reducing nitrates. Many 

 strict aerobic bacteria are capable of acting anaerobically in the pres- 

 ence of nitrates. Intensive aeration inhibits the process of nitrate re- 

 duction. The reaction of the medium has an important influence in 

 determining whether nitrates are reduced to nitrites or ammonia; an 

 alkaline reaction favors the first process and an acid reaction the 

 second. 



Klaeser used a medium having the following composition : 



KN0 8 2 grams NaCl 0.1 gram 



Glucose 10 grams MgS0 4 0.3 gram 



K 2 HP0 4 1 gram FeCl 3 0.01 gram 



CaCl 2 0.1 gram 



Other media, with and without peptone, but containing nitrates, can also be 

 used for demonstrating nitrate reduction by bacteria. The formation of nitrites 

 from nitrates has been suggested as a test in characterizing bacteria. 16 



The following organisms can be recorded as capable of reducing 

 nitrates to nitrites: Bad. coli, Bad. vulgare and allied species, Bad. 

 prodigiosum, Bad. putidum, Bad. fluorescens, Bad. pyocyaneum, Bad. 

 herbicola, Bac. subtilis and allied species, Bac. vulgatus, Bac. mycoides, 

 Micr. pyogenes, Mycobad. phlei and other mycobacteria, B. porticensis 

 and others. Some of these organisms, such as Bad. coli, are also cap- 

 able of forming hydrogen. 17 The products formed from the reduction of 

 the nitrate depend largely upon the composition of the medium and 

 oxygen tension. 



Organisms reducing nitrates to ammonia. Marchal 18 was one of the 

 first to demonstrate that certain bacteria (Bac. mycoides) are capable of 

 reducing nitrates to ammonia, with the intermediate formation of ni- 



15 Maassen, 1901 (p. 181). 



16 Conn, H. J., and Breed, R. S. The use of the nitrate-reduction test in char- 

 acterizing bacteria. Jour. Bact. 4: 267-290. 1919. 



17 Maze, P. Les phdnomenes de fermentation sont les actes de digestion nou- 

 velle demonstration apport£e par l'etude de la devitrification dans le regne vegetal. 

 Ann. Inst. Past. 25: 289-312, 369-391. 1911. 



18 Marchal, E. The production of ammonia in the soil by microbes. Agr. 

 Sci. 8: 574. 1S94; Centrbl. Bakt. II, 1: 758. 1895. 



