110 BACTERIOLOGY. 



pounds into the latter. Hence, the nitrite-forming or ni- 

 troso-bacteria must be supplied with an ammonium salt, 

 whereas the nitrate-producing' forms, or nitro-bacteria re- 

 quire a salt of nitrous acid. 



The formation of nitrites and of nitrates in the soil is, 

 therefore, as Pasteur pointed out in 1862, a process analog- 

 ous to acetification. The production of acetic acid and of 

 nitric acid is due to oxidation changes, which are not purely 

 chemical, as was at one time supposed. These changes are 

 dependent upon the presence of certain organisms. Nitri- 

 fication is, therefore, a fermentation analogous to that of 

 acetic acid. It is not one organism but rather a large 

 number, or group of organisms, which possess the power of 

 inducing these fermentative changes. 



A striking contrast to the action of these organisms is 

 that of the de-nitrifying bacteria. The latter possess 

 remarkable reducing powers. They convert the nitrates 

 into nitrites, and continue the reduction until all, or nearly 

 all, of the nitrogen has been set free, as such. This appar- 

 ent waste of nitrogen may be considered as being balanced 

 by the activity of the bacteria which are present in the root 

 nodules of leguminous plants. These organisms are able 

 to assimilate, or fix the atmospheric nitrogen, and trans- 

 fer it to their host. There are, therefore, four groups of 

 bacteria according to their action on nitrogen and its 

 inorganic compounds. 



1. Those which cause a fixation of free nitrogen. 

 2. Those which produce nitrites out of ammonia. 

 3. Those which produce nitrates out of nitrites. 

 4. Those which reduce nitrates to nitrites and to free 

 nitrogen. 



A further illustration of the reducing action of bac- 

 teria is seen in the manufacture of indigo. The indigo does 

 not exist, as such, in the plant, but rather as a glucoside. 



