130 PLANTS AND ANIMALS 



reach. We also learned that they may cause disease but 

 that they can be killed by heat, bright sunshine and by 

 antiseptic washes. 



The bacteria are much smaller than the yeasts and 

 can only be seen by the use of a very powerful micro- 

 scope. They need moisture and a little warmth in order 

 to grow, but they can live on almost any kind of food. 

 Each increases in numbers ver^ rapidly,- if supplied with 

 food, in a manner somewhat similar to yeasts. Instead 

 of budding, however, each becomes longer and separates 

 in the middle into two bacteria. This method of increase 

 is called fission. Under favorable conditions a single 

 bacterium will produce seventeen million bacteria in 

 twelve hours. 



Bacteria are the cause of many of the more delicate 

 flavors of food. Thus butter and most kinds of cheese 

 owe their peculiar flavor to the growth of bacteria. It 

 is due to bacteria that the nitrogen of the air can be 

 changed into food which is suitable for plants. See 

 Section 31. There are also several beneficial changes in 

 substances for which we must thank the bacteria. It is 

 probable that there could be no life if it were not for the 

 effects which are produced by bacteria. Thus we see 

 than they are our friends. 



Some kinds of bacteria are very harmful, and we 

 should learn how to protect ourselves from them. Be- 

 fore studying how to preserve our health let us learn 

 what the bacteria do to materials upon which they feed. 



51. Souring and Decay. 



When bacteria have been present in food for some 

 time, they change it into substances which have bad 

 odors and flavors. The food is spoiled and we call this 



