14 BACTERIOLOGY. 



rain becomes mixed with the soil, the number of bac- 

 teria present is very large. 



Foods become contaminated with bacteria in a 

 variety of ways. Vegetables always have the soil 

 bacteria on their surface. Meats if exposed to the 

 air take up bacteria from the dust. The surfaces of 

 fruits become contaminated with bacteria in the same 

 way. In order to diminish the contamination of foods 

 as much as possible, ordinances are in force in .many 

 cities that require meats, fruits, candies, etc., to be 

 covered with glass when displayed for sale. 

 tkXi nC o~f With bacteria so widely distributed on the earth, 



erl f the question arises as to their use or function in the 

 world. We are accustomed to think of bacteria solely 

 as the cause of disease, and offhand we would say that 

 this was their chief function. This is not true by any 

 means, for instead of being harmful to life they are 

 very beneficial; in fact, life could not be maintained 

 without them. The causation of disease is a function 

 limited to a small group of micro-organisms, and is 

 of lesser importance. The much more important use 

 of bacteria relates to their ability to produce sub- 

 stances called ferments or enzymes, which have the 

 property of reducing complex organic compounds into 

 simpler compounds and chemical elements. 



The plants which form the food of animals would 

 soon be exhausted unless they could obtain proper 

 nutriment to sustain life and reproduce their kind. 

 They live mainly upon carbon and nitrogen in the form 

 of nitrates, which would soon be consumed from the 



