CHAPTER IV 

 CLASSIFICATION OF BACTERIA 



Classification According to Nutrition. — An illuminating classification 

 of bacteria has been based on their mode of life, where three biologic 

 groups may be recognized : the prototrophic, the metatrophic and the 

 paratrophic bacteria. The prototrophic bacteria, which include the 

 nitrifying bacteria, bacteria of root nodules, sulphur and iron bacteria 

 and erythrobacteria, are those which either require no organic com- 

 pounds for their nutrition, or which given a small amount of organic 

 carbon can derive all of their nitrogen from the atmosphere, or which 

 with a minimum of organic matter can derive energy by breaking up 

 inorganic bodies. 



The sulphur bacteria live in sulphur springs where hydrogen sul- 

 phide (H2S) is formed by putrefaction of dead animals and plants. 

 The sulphur bacteria in such places form a white furry growth on the 

 rotting vegetation. Here the H2S is attacked and water and sulphur 

 are formed, H2S + O = HjO + S. The sulphur is deposited in the 

 living cells of the bacteria as yellow amorphous granules, which impart 

 to the organism a yellow color. To explain the facts observed, we need 

 assume only that the protoplasm increases the oxidizing power of the 

 atmospheric oxygen and renders it active. The conversion of H2S 

 into water and S gives 71 calories and the further oxidation of the freed 

 sulphur into sulphuric acid 2109 calories. The fact that the sulphur 

 bacteria can live without organic compounds together with their inability 

 to live without sulphur indicates that it is the oxidation of the sulphur 

 alone which takes the place of respiration in other organisms. 



The ferrobacteria live in stagnant pools in marshy places. On 

 such pools of water, we find a greasy scum of ferric hydroxide Fe(OH)3 

 together with organic matter and some phosphate of iron. The ferric 

 compounds are reduced by the action of reducing substances formed by 

 putrefaction to the ferrous state which are dissolved by carbon dioxide 

 CO2 and unite also with it to form ferrous carbonate. The atmospheric 

 oxygen can convert this carbonate back to ferric hydroxide, but Wino- 



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