Microorganisms in Relation to Man 585 



But lactic acid is exactly the food material for the mold, oidium 

 lactis, which begins to develop and by consuming the acid main- 

 tains the condition in which the acid-producing bacteria thrive. 

 Another instance is found in the relations of certain soil bac- 

 teria. Protein substances in decaying organic matter are 

 changed by bacteria into ammonia. A different species of bac- 

 teria attacks and uses the ammonia, oxidizing it to form com- 

 pounds which are called nitrites. As soon as nitrites are 

 present in the soil, another species of bacteria begins to develop, 

 and using the nitrites as food material, combines them with 

 oxygen and produces nitrates. These nitrogen compounds 

 are at once available to green plants. Microorganisms of this 

 species are called the nitrate bacteria. This helps in under- 

 standing the production of the great beds of sodium nitrate or 

 saltpeter in Chile and Peru from the accumulations of seaweeds 

 in inland seas. The seaweeds were gradually decayed and 

 changed into ammonia, then into sodium nitrite, and finally 

 into sodium nitrate. 



These examples indicate ways in which many microscopic 

 plants are interdependent or cooperative among themselves. 

 They may further suggest how chlorophyll-bearing plants are 

 dependent upon the work of microorganisms, and how we who 

 depend upon the green plants for food and clothing are also 

 dependent upon the invisible multitudes of bacteria which are 

 growing everywhere. 



SUMMARY 



Bacteria like molds and yeasts are dependent plants having no 

 chlorophyll. 



Many kinds of bacteria are useful ; a few are pathogenic. 



Pathogenic bacteria cause such diseases as tuberculosis, pneu- 

 monia, diphtheria, typhoid fever, cholera, and tetanus. 



Some bacteria produce putrefaction and decay, others prepare the 

 nitrates necessary for plant growth. 



Bacteria may be classified on the basis of their source of food into 

 three groups : (1) saprophytes, (2) parasites, (3) independent bacteria. 



Bacteria are also classified on the basis of their form and appear- 



