66 THE RELATION OF BACTERIA TO DISEASE 



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 pharynx; 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 

 unqualified means extracellular toxins, while intra- 

 cellular poisons are specifically called endotoxins. 

 Some bacteria (cholera for example) develop both 

 kinds. 



The local gross effects of bacterial invasion are 

 expressed in inflammation, which is greatest in those 

 which act by their mechanical presence in a confined 

 locality, usually aided by some of the posions men- 

 tioned above. 



