86 BACTERIOLOGICAL EXAMINATION OF WATER. 



The differences observed appear to be due to the great porosity 

 of earth ; as compared with a fluid the surfaces exposed in soil 

 are enormous, and the nitrate ferment is never repressed by a 

 want of oxygen unless the ground is saturated with moisture or 

 dosed with too much ammonia. 



The power of reducing nitrates to nitrites and ammonia, the 

 converse of the oxidation processes just considered, appears to be 

 common to many bacteria. The B. fluorescens liquefaciens, 

 B. mycoides, and the varieties of B. arborescens and B. nubilus, 

 which I have isolated from water, reduce nitrates very vigorously. 

 Special denitrifying bacteria, however, have been isolated by 

 Stutzer, Bum, and Jensen. 



Bacillus Denitriflcans I. 



This organism was isolated by Stutzer and Burri from horses' 

 faeces. It is a small motile bacillus, which does not form spores. 

 In gelatine plates there is a limited growth on the surface, and 

 small round colonies appear in the depth ; the gelatine is not 

 liquefied. Stroked over a gelatine-slope it produces a dull 

 bluish grey layer. On agar plates the colonies are large, round, 

 and very thin. On an agar slope there is a thin grey layer. On 

 potato, especially at blood temperature, there is a limited 

 reddish-brown growth. In symbiosis with B. coli or B. typhosus 

 it reduces nitrates very vigorously. 



Bacillus Denitriflcans II. 



This bacillus was isolated by Stutzer and Burri from straw. 

 It is a small motile bacillus, which does not form spores, and 

 grows best at blood temperature. The surface colonies in 

 gelatine plates are very characteristic ; they appear as large, 

 coarse growths, marked with radial ridges, which branch as they 

 pass outwards and then bend in and interlace at the margin ; 

 the gelatine is not liquefied. On agar a similar characteristic 

 growth is seen. On potato there is a slimy, reddish-coloured 

 growth. In broth a pellicle forms on the surface. This 

 organism acts in symbiosis with B. coli, in the same manner as 

 Bacillus I. According to Weissenberg, the B. coli reduces the 

 nitrates to nitrites, which are then acted upon by the denitrifying 

 bacilli. 



