THE RELATION OF BACTERIA TO DISEASE 61 



means the peculiar expression of bacterial disease has 

 been found, and thus it becomes possible to separate 

 those diseases which are wholly due to the bacteria 

 themselves and those principally arising from the bac- 

 terial poisoning. 



Bacterial Toxins. Diphtheria is a disease wherein 

 the bacteria reside and grow on a free surface such as 

 the mouth; but their poisons are absorbed and carried 

 in the blood stream, thus producing the peculiar 

 symptoms of the disease. If, however, this toxin is 

 taken, entirely free of diphtheria bacilli, and injected 

 into animals, the same results can be obtained so 

 far as the symptoms are concerned. This is likewise 

 true of tetanus. 



For the development of typhoid fever and septicemia 

 it is necessary that the bacteria themselves should 

 circulate in the blood stream. The reason for this is 

 that while the poisons of the diphtheria bacilli are 

 soluble in fluids and separable from the germs, the 

 poisons of the typhoid bacillus, for instance, remain 

 within the body of the germ and are only effective when 

 the cell dies and disintegrates. The former poisons are 

 called extracellular toxins, and the latter intracellular 

 toxins or endotoxins. In practice the word toxin unquali- 

 fied means extracellular toxins, while intracellular 

 poisons are specifically called endotoxins. Some bac- 

 teria (cholera, for example) develop both kinds. 



Bacterial poisons, it might be said, usually express 

 some definite predilection for special organs or tissues. 

 For instance, the tetanus toxins attack the brain. The 

 streptococci attack red blood cells, and the typhoid 

 bacillus settles in the lymph glands of the small 

 intestine. 



