CH. XI J BACTERIA IN THE SEA 261 



can also use as food materials the nitrates and nitrites and carbon 

 dioxide, but the activity of the plant, so far as its carbon assimila- 

 tion is concerned, is bound up with the possession of chlorophyll, 

 which substance enables it to utilise the energy of sunlight. Now the 

 prototrophic bacteria have no chlorophyll ; they can live and grow 

 in the dark ; and they can synthesise the complex proteid substance 

 of their living bodies from nitrates, nitrites, carbon dioxide and a 

 few simple mineral salts. No other organisms can do this. They 

 live in the open and are never parasitic. 



(2) The metatrophic bacteria. Most bacteria belong to this 

 division. They live upon the dead organic substance of plants and 

 animals. They can form enzymes or ferments which bring about 

 important chemical changes in many substances. They have many 

 sources of food, but this is always highly complex compared with 

 the food-stuffs of the prototrophic germs. They may live as para- 

 sites within the animal body or on the surfaces of the bodies of 

 both plants and animals. They are sometimes parasites but they 

 can also live in the open. 



(3) The paratrophic bacteria. These also live upon the dead 

 or diseased organic matter of the plant and animal organism, 

 and they are always parasites inhabiting the tissues or cavities of 

 the living body. Like the metatrophic bacteria they demand as 

 food-stuffs the highly complex substances of organised matter. 

 They never live in the open. Most disease-producing bacteria 

 belong to this division. 



The putrefactive bacteria belong to the metatrophic group. 

 When the body of a plant or animal dies it may, or it may 

 not, decompose. If the dead body is kept at a low temperature (as 

 in the case of the famous Siberian mammoth) it will not putrefy. 

 If it is kept at a moderately high temperature (say 50° C.) it will 

 not putrefy. If again it is permeated with certain substances, 

 such as corrosive sublimate, or carbolic acid (antiseptics), neither 

 will it putrefy. Decomposition is always due to the living activity 

 of metatrophic bacteria, and in the above circumstances the 

 bacteria are either destroyed, or their activity is inhibited or is 

 arrested. 



When the proteid substance of the body is decomposed by boil- 

 ing acid, or caustic potash, or barium hydrate, certain substances 



