HOW BACTERIA PEODUCE DISEASES 497" 



How bacteria produce diseases and other effects. — In the 

 act of obtaining their food bacteria may bring about chemi- 

 cal changes in the material from which they take it, thus 

 transforming that material into new and simpler com- 

 pounds. 



Again, they may give off as wastes, materials which are 

 poisonous to the host upon which they live. Such poisons 

 are called toxins. 



Finally, they may by rapid reproduction become so 

 numerous as to clog effectually the part of the host where 

 they are present, and thus interfere with the normal 

 action of that part. 



All bacterial effects may be traced to one of these three 

 causes. For example, the souring of milk is brought about 

 as follows: The lactic bacteria found in milk break up the 

 sugar in the milk in the attempt to obtain food and trans- 

 form it into lactic acid. This acid causes the proteid part of 

 the milk to coagulate or curdle, and gives the sour taste and 

 odor. Again, the bacteria which collect in meat break up 

 the complex proteid matter into simpler compounds and 

 thus reduce it to simple mineral compounds. Both of 

 these examples illustrate the first mentioned effect of 

 bacteria. 



The action of the diphtheria bacterium illustrates the 

 second method. In this case, the bacterium gives off a 

 toxin, or poison, which enters the blood and disturbs the 

 action of the organs of the body. In the case of diphtheria, 

 a substance has been found which if injected into the blood 

 destroys the effect of the toxin, and is called on that account 

 an anti-toxin. 



The formation of tubercles by the massing of the tuber- 

 culosis forms in the lungs, thereby breaking down the 



EDDY. PHTS 32 



