CHAPTER XII. 



THE RELATION OF BACTERIA TO DISEASE. 



The action of bacteria on living tissues. It has been 

 stated that bacteria are divided into two distinct groups : one 

 saprophytic or those living on dead organic matter, and the 

 other pathogenic or those which live in the tissues of living 

 plants and animals. The bacteria that acquire, if they did 

 not originally possess it, the ability to live and multiply in 

 living tissues produce as a result of their multiplication 

 changes of more or less significance in the tissues of the host. 

 Smith x has pointed out that certain bacteria which have 

 acquired the habit of parasitism in animal tissues elaborate a 

 poisonous toxin which primarily appears to be for the pur- 

 pose of assisting the bacteria to overcome the resisting forces 

 of the living tissues but which perhaps unintentionally on the 

 part of the organism poisons their host. Bacteria which have 

 already attained a high degree of efficiency of parasitism, such 

 for instance as those of tuberculosis and leprosy, are able to 

 live and to multiply in the tissues in opposition to the resist- 

 ing forces of their host. In these cases the diseases they pro- 

 duce are due largely to the destruction of the tissues, while in 

 the former case the symptoms and often fatal results are due 

 to the immediate effects of the toxins themselves. 



Smith has explained the general phenomenon of infection 

 to be that the tendency of all invading microorganisms in 

 their evolution toward a more highly parasitic state is to act 

 solely on the defensive while securing opportunity for multi- 

 plication and escape to another host. By tendency is meant 

 a general slow movement through long periods of time. The 

 following data are in its favor : 



1. "The production of diffusible toxins survives para- 

 sitism indefinitely, and is readily brought about in cultures. 

 1 Smith. American Medicine, Vol. VIII (1904) p. 711. 



