BACTERIA 137 



rest of the plant and several steps in the tanning of leather, curing 

 tobacco, and preparing sponges, are other processes which depend 

 on bacteria. 



Harmfu Bacteria. On the other hand, tuberculosis, which causes 

 one-seventh of all the deaths in the world, is due to the attack of 

 a bacterium. At least fifty per cent of all deaths are due to this and 

 other bacterial diseases, of which the following is a partial list. 



tuberculosis tooth decay anthrax 



erysipelas pneumonia cattle fevers 



leprosy ptomaine poisoning grippe and colds 



syphilis typhoid fever lockjaw (tetanus) 



diphtheria eye diseases cholera 



whooping cough 



Often when bacteria attack nitrogenous foods, poisonous sub- 

 stances, called ptomaines, are produced. These sometimes cause 

 illness or death when such food is eaten. Some serious plant diseases 

 or " blights " are caused by bacteria and result in great crop losses. 

 Bacteria were discovered by Pasteur in his reaserches along this 

 line. 



Defences against Bacteria. With this formidable list in view, it 

 is evident that we ought to know how to prevent these bacteria 

 from attacking our bodies and how to combat and destroy them 

 when they obtain a foothold in our systems. 



Skin. Our first line of defence against these ever-present enemies 

 is our skin, and the mucous membranes which line the inside of the 

 body. If they are clean, whole, and healthy, few bacteria can get 

 inside our defences. 



NATURAL RESISTANCE 



If they break through this outer breastwork, the bacteria have 

 to face the second line of defence, which is the natural resistance of 

 a healthy body to any harmful invader. This second line is de- 

 fended by the white corpuscles in the blood, which actually de- 

 vour some of the disease germs, and also by antitoxins, which 

 overcome the poisons made by the bacteria, and which are produced 



