462 MAN CONTROLS HIS ENVIRONMENT FOR HEALTH 



the food tube. Some in the food tube are believed to be useful, 

 some harmless, and some harmful; others in the mouth cause 

 decay of the teeth, while a few species may cause disease. Such 



disease-causing bacteria are 

 called pathogenic. 



It is known that bac- 

 teria, like other living 

 things, take in food, form 

 organic wastes within their 

 own bodies, and give off 

 some of them. These 

 wastes, called toxins, are 

 poisonous to the host on 

 which the bacteria live, 

 and cause the symptoms 

 of certain diseases. Each 

 species of bacteria forms 

 Roy M.Aiien its own specific toxin, and 



A microphotograph of a Petri dish containing a pure this has a Specific action 

 culture of bacteria that cause cholera. ^ 



on the body, causing the 

 symptoms of a specific disease. As bacteria can multiply rapidly 

 in the body, they may become very numerous before the body 

 defenses gain control of the situation. When the bacteria die, as 

 they may in great numbers during the progress of the disease, 

 their bodies break down, and the released protoplasmic constit- 

 uents, particularly the proteins, separate from each other and 

 split into smaller and smaller molecular groups, as do the proteins 

 when changed to amino acids during digestion. These split pro- 

 teins, as they are called, are extremely poisonous to the body 

 tissues and act as toxins in the body, causing many of the charac- 

 teristic symptoms of disease. 



Some bacteria break down the body tissues, besides producing 

 toxins. They may destroy the intestinal lining, or destroy the 

 blood corpuscles, or break down tissues in wounds, thus causing 

 specific symptoms of disease. 



It was estimated not many years ago that bacterial diseases 

 caused annually almost 50 per cent of the deaths of the human 



