[Chap. XLII THE BIOLOGY OF BACTERIA 523 



time. They often develop toxic substances in the medium that inhibit 

 further growth. Their food, water, oxygen, and salt supply is limited to 

 the film immediately surrounding them. Colonies of bacteria cannot 

 become very large because the movements of substances through the 

 mass is limited by the rate of diffusion. In nature other environmental 

 factors such as light, moisture, and temperature rarely remain favorable 

 for long periods. Bacteria are also consumed by many microscopic 

 animals. 



The protoplasm of a bacterial cell may contract into a rounded mass 

 at one end, or in the middle, with the formation of a secondary wall; 

 this mass is termed a spore. It contains less water, is less likely to be 

 injured by drying, and will endure greater extremes of temperature and 

 greater concentrations of poisons than the ordinary bacterial cell. It is 

 because the spores of certain bacteria can withstand a temperature of 

 boiling water that steam pressure is used in sterilizing cans of com, 

 beans, peas, and other vegetables. Most of the bacteria that cause dis- 

 eases do not form spores. Those that cause botulism and tetanus are 

 exceptions. 



Bacteria and sanitation. We often go to considerable trouble to prexent 

 decay; but we owe our continued existence on the earth to the fact that 

 bacteria (and fungi) remove the dead bodies of plants and animals that 

 would otherwise have accumulated on the earth. Together with the fungi 

 they secrete enzymes which digest the highly complex organic sub- 

 stances composing the bodies of plants and animals into simpler sub- 

 stances. By repeated oxidations these substances are ultimately changed 

 into carbon dioxide, water, and inorganic salts. These are again available 

 to green plants. 



The sewage that is drained into rivers is digested and oxidized by 

 bacteria to simple harmless compounds. Our great cities, where thou- 

 sands and sometimes millions of people are crowded into a small area, 

 must have enormous sewage-disposal works in which aeration and other 

 conditions are made favorable for a more rapid growth of the bacteria of 

 decay, and the quick destruction of the sewage. This prevents the pollu- 

 tion of streams and lakes into which the sewage would otherwise be 

 carried. 



The modern processes of filtering and sterilizing the water supplies 

 of cities not only remove sediment, but also eliminate the bacteria of 

 disease. The processes include adding minute quantities of alum and 

 chloride of lime to the water, and then filtering it through sand. For 



