LEO F. RETTGER 225 



mixture of amino acids; and, again, some cannot be made to grow in any known mix- 

 ture of amino acids, but require for their normal development special material like 

 hematin (//. influenzae) or substances of unknown composition which are present in 

 blood or blood serum (some streptococcus and pneumococcus forms). 



UTILIZATION OF AMMONIA AND ATMOSPHERIC NITROGEN 



Both ammonia production and ammonia utilization are properties common to 

 most bacteria, if indeed not all. These processes are frequently carried on simultane- 

 ously. The ammonia is apparently utilized directly. 



Numerous organisms will develop abundantly in media in which the only source 

 of nitrogen is ammonia, in the form of a soluble ammonium salt. Proskauer and Beck' 

 found that good growth of the tubercle bacillus (human type) can be obtained in a 

 medium containing ammonium salts of di-basic and tri-basic acids. This observation 

 has been verified frequently. Another example is the medium of Ayers and Rupp' for 

 the cultivation of the colon-aerogenes group. The nitrogen is supplied here in the 

 form of sodium ammonium phosphate. 



Free atmospheric nitrogen can be used by some organisms as a source of nitrogen. 

 The conversion of free nitrogen into ammonia for cell use may be demonstrated easily 

 by the Azotobacter genus and CI. pastorianum. It appears that non-symbiotic ni- 

 trogen fixation among aerobic bacteria is not uncommon; but the action is as a rule 

 so weak and variable as to have little practical significance. 



A process of free nitrogen utilization which is of enormous economic importance is 

 that of the Rhizobium or root-nodule type of or gSLuism —Rliizob mm radicicola. This 

 depends on a close association with the roots of leguminous plants, and takes place 

 within the root tubercles of the legumes. Little is known of the chemical process, but 

 it would seem that ammonia must be an important intermediate product of me- 

 tabolism. 



Denitrification is a property possessed by a considerable number of organisms, 

 particularly of soil origin. Complete denitrification results in the formation of nitro- 

 gen or of nitrous oxides. Indirect or incomplete reduction of nitrates gives rise to 

 nitrites or ammonia. 



Another process engaged in by a very limited number of organisms is that of ni- 

 trification. This involves two separate processes: (i) the oxidation of ammonia to 

 nitrous acid, accomplished by two highly specialized species, the Nitrosomonas and 

 Nitrosococcus of Winogradsky; and (2) the further oxidation of the nitrous acid to 

 nitric through the agency of the Nitrobacter of Winogradsky. 



These two processes may be indicated by the following equations: 



(i) 2NH, +30. = 2HNO.-f2HA 



(2) 2HN0.-h 0. = 2HN03. 



While the nitrification plays an important economic role in soil fertility, it would 

 seem that the chief immediate benefit which the bacteria themselves derive from it is 



' Proskauer, B., and Beck, M.: Zlschr.f. Hyg. u. Infekiionskrankh., 18, 128. 1894. 

 - Ayers, S. H., and Rupp, P.: /. Bad., 3, 433. 1918. 



