498 BACTEEIA AND SAJSTITATION 



tissues and clogging the air sacs, is an example of the third 

 method of action. Frequently a given bacteriimi combines 

 these methods of effect. In every case, however, the or- 

 ganism produces its effect through some result of its meta- 

 bolism, and these secondary effects, such as poisoning or 

 clogging, decay, and the like, are the result of the attempt of 

 one organism to take its food from another. 



Beneficial bacterial action. — It must not be thought 

 that all bacteria are harmful. If it were not for decay 

 of waste materials, for example, the farmer would be un- 

 able to raise any crops. Decay in his case means the trans- 

 formation of dead plant and animal matter into simpler 

 compounds, such as are acceptable as food to growing 

 plants, and without bacteria the supply of such food in the 

 soil would be soon exhausted. Again, in the manufacture 

 of many dairy products, such as butter and cheese, the bac- 

 teria perform a very important part in the manufacture. 

 Certain forms of bacteria have also been found which, if 

 introduced into the soil with certain plants, can transform 

 the nitrogen of the air into soluble nitrates which the 

 plants can use, and such forms are extremely valuable. 



Harmful bacterial action. — The forms of bacteria which 

 are most harmful to man are those which produce disease, 

 and the following list gives us some idea of the diseases 

 which are now known to be due to bacterial action: 



Bubonic plague Diphtheria Lockjaw 



Asiatic cholera Erysipelas Consumption 



Fowl cholera Glanders Typhoid fever 



Hog cholera Boils and carbuncles 



The forms of bacteria which are beneficial in causing de- 

 cay of waste material may be harmful to food supplies. The 



