Bacteria 205 



velop in a very short time. Under conditions 

 unfavorable for growth, some species may form 

 within their interior dense masses which are called 

 spores. These resemble the seeds of higher 

 plants in their function of distributing the spe- 

 cies and in preserving life through intervals of 

 time unfavorable for continuous multiplication. 



Bacteria may be classified in several ways, 

 but for the discussion of cleanliness and sanita- 

 tion, the simplest division is into the harmless 

 and the injurious. The harmless forms live 

 mostly on dead organic matter, causing nitrifi- 

 cation, fermentation, and putrefaction ; they 

 break down the more complex organic com- 

 pounds into simpler ones, so that they can be 

 used again -as food for plants. Familiar ex- 

 amples of this are seen in the decay of meat 

 and vegetables. This class is more numerous, 

 much hardier than the other, and comprises an 

 overwhelmingly large proportion of the bacteria 

 in nature. 



Bacteria are found almost constantly in water, 

 in soil, and in air. Consequently they are pres- 

 ent in all our food, except that which has been 

 heated to kill them. Certain bacteria are nor- 

 mal inhabitants of the mouth, throat and intes- 

 tines, while others find suitable conditions for 

 growth on the skin and in the accumulation of 

 substances excreted in the perspiration. 



