30 A MANUAL OF BACTERIOLOGY 



broken up ; first they become liquefied, peptone-like 

 bodies being produced ; these are then further acted upon 

 and we get alkaloidal substances in small quantity, indole, 

 skatole, leucin, and tyrosin and amino-acids, valerianic 

 acid, volatile fatty acids, lactic acid, etc. 



These changes are brought about by numbers of 

 organisms, among which the varieties of Proteus (formerly 

 known as Bacterium termo), B. mesentericus, B. mycoides, 

 B. fluorescens liquefaciens, and B. putrificus are the more 

 important. 



The nitrogenous compounds are then further acted upon 

 and ammonium salts are formed. According to Emile 

 Marchal, ammonisation takes place essentially under the 

 influence of microbes living in the upper layers of the soil. 

 The Bacillus mycoides is one of the most energetic of these, 

 and seems to play a double role, being ammonising in the 

 presence both of nitrogenous organic substances and of 

 nitrates. Urea is ammonised especially by the Micro- 

 coccus urece. 



II. Nitrosation. The ammoniacal salts are next con- 

 verted into nitrites. The nitrous organisms can probably 

 attack nitrogenous organic substances such as asparagine 

 and milk, but only feebly, milk being much more rapidly 

 nitrified when the nitrous organisms are mixed with other 

 species. The organisms bringing about this change are 

 short, stumpy, motile bacilli with single polar flagella 

 which are grouped under the generic name of Pseudo- 

 monas. 



III. Nitratation. These nitrites are then converted 

 into nitrates. The " nitric " organisms are minute non- 

 motile bacilli known as Nitrobacter. 



Stages II and III are brought about by different species, 

 the nitric organisms having no effect whatever on ammonia, 

 but acting only after this has been oxidised into nitrous 

 acid by the nitrous forms. 



