Autotrophic Bacteria 61 



Autotrophic Bacteria 



Autotrophic hactcria arc characterizect by certain physiological 

 properties that differentiate them sharply from all the other bacteria. 

 The principles originally laid down by Winogradsky for the growth 

 of these bacteria still hold today with only slight modifications. The 

 characteristic properties of these organisms can be summarized as 

 follows : 



1. Their development in nature takes place in strongly elective 

 mineral media, which contain specific oxidizable inorganic sub- 

 stances. 



2. Their existence is connected with the presence of such inor- 

 ganic elements or simple compounds, which undergo oxidation as 

 a result of the life activities of the organisms. 





Fig. 27. Nitrite-forming bacterium, Nitrosomonas europea (from Winogradsky). 



3. The oxidation of the inorganic substances supplies the only 

 source of energy for the growth of these organisms. 



4. They do not need any organic nutrients either for cell synthesis 

 or as a source of energy. 



5. They are almost incapable of decomposing organic substances 

 and may even be checked in their development by certain com- 

 pounds. 



6. They use carbon dioxide as an exclusive source of carbon, 

 which is assimilated chemosynthetically. 



The isolation of autotrophic bacteria forms one of the most fas- 

 cinating chapters not only in the history of soil microbiology but 

 also in the history of microbiology as a whole. 



Nitrifying Bacteria. Among the autotrophic bacteria, the nitri- 

 fying organisms have received the greatest consideration, because of 

 the importance of the process of nitrification in the soil, in composts, 

 in sewage-disposal systems, and in fresh and salt waters. During the 

 last three decades of the nineteenth century, the elucidation of the 



