318 ' ANIMAL BIOLOGY 



There are countless different kinds of bacteria; they 

 live under the most diverse conditions, and are capable 

 of subsisting upon a great many kinds of food. Many 

 forms cause the decay of the bodies of higher organisms. 

 Whenever an organic body putrefies or decays, it may be 

 found to be teeming with multitudes of bacteria. If these 

 minute organisms are excluded from an organic body it 

 may be preserved for a long period. Canning fruits, 

 vegetables and meats is essentially a device for keeping 

 these articles free from bacteria. The heating of canned 

 goods kills whatever bacteria may have been present in 

 them, and the sealing prevents the access of others. 

 Once introduce bacteria and the substances soon decay. 



Many articles are kept from spoiling by means of 

 preservatives, or substances which either kill bacteria or 

 check their growth. Up to a point which varies with 

 different species, bacteria multiply more rapidly as the 

 temperature is increased, and they are kept from multipli- 

 cation at a temperature at or near the freezing point. 

 Cold storage therefore keeps them from attacking meats 

 and other articles of food; hence our cold storage plants, 

 refrigerator cars and ice chests. 



Bacteria are usually not killed by freezing, however, 

 and many kinds will endure a temperature of over iooC. 

 below zero without losing their vitality. Ice may harbor 

 the germs of many diseases and in particular those of 

 typhoid fever which are especially hard to kill. Boiling 

 kills most bacteria, but the spores of many species will 

 resist even boiling for a certain period. 



Bacteria are almost universally distributed in water, 

 soil and all sorts of organic material. Owing to their 

 minute size they are capable of being easily carried through 

 the air. If a bit of beef broth or vegetable infusion is 

 left exposed to the air for a few moments it will become 



