66 BACTERIOLOGY FOR NURSES 



pounds in the form of nitrates (-NOa) are the most important. 

 Potassium, phosphorus, and certain other elements furnish a part 

 of the plant food, but they are of minor importance and for the 

 sake of simplicity may be left out of consideration here. The 

 plant takes from the soil and the air the material it requires, and 

 by means of the energy furnished it by the sun's rays it builds 

 these simple substances into more complex ones such as sugars, 

 starches, proteins, and fats. This brings us to step B. From 

 these products of plant life animals obtain their food. Standing 

 at the top of the circle C, they are incapable of utilizing the simpler 

 compounds, but are wholly dependent on the products manu- 

 factured by the plants. The complex food then is eaten by ani- 

 mals and becomes part of their bodies. As a result of muscular 

 activity part of the carbon and oxygen is speedily converted into 

 carbon dioxide and exhaled into the air. The nitrogen compounds 

 are in part reduced at once to urea and excreted as such, while 

 most of the remainder is retained to build up body tissue. When 

 the body dies this nitrogenous material is far too complex for 

 immediate use by the plants, and in order to complete the cycle 

 it must be reduced to simpler compounds. It is at this point D 

 that bacteria form a link in the chain. Always present in the 

 air, soil, and water, they seize hold of any organic substance that 

 can serve them as food and break it into simpler and simpler com- 

 pounds until finally it is in the right condition to serve again as 

 plant food. 



Many species take part in this putrefactive process. Bacillus 

 proteus, bacillus subtilis, and the colon bacilli are amongst the 

 most important. Pathogenic bacteria are for the most part killed 

 during the early stages of decomposition. 



The breaking down of the highly complex protein substances 

 into simpler and more stable compounds is spoken of as minerali- 

 zation or denitrification. Sometimes it happens that the process 

 is carried so far that the products are too simple even for plant 

 food. From decaying animal bodies part of the carbon is returned! 

 to the air as carbon dioxide and part combines with the alkali 

 in the soil to form carbonates. The nitrogen compounds that 



