METHODS AHD CHANNELS OF INFECTION. 535 



The cells producing these toxic substances are only indirectly responsible 

 for the infections and that it is the toxins themselves which produce the 

 pathogenic effect on the body. 



ENDOTOXINS. Many of the pathogenic bacteria and some of the 

 protozoa do not secrete their toxins outside the cell wall but hold 

 them within the wall in combination with the protoplasm. They do 

 not liberate these substances until the microorganisms die and are dis- 

 integrated. Such toxic substances are called endotoxins to distinguish 

 them from those secreted from the cell, namely, the soluble toxins. Two 

 of the best examples of pathogenic bacteria of this type are the Msp. 

 comma of Asiatic cholera and B. typkosus of typhoid fever. 



Toxic BACTERIAL PROTEINS. There are some bacteria and other 

 parasitic cells which produce a small amount of endotoxin and in certain 

 instances some soluble toxin but not enough of either of these substances 

 to account for the toxicity of the microorganism. It has been found 

 that, when organisms of this character are ground up and washed to 

 free them of their endotoxin and are washed free of all soluble toxins, 

 they are still toxic. It has been shown that this toxicity is due to the 

 protein substances of the cell. The Bact. tuberculosis and the Bact. 

 mallei, of glanders, are two notable examples of microorganisms of this 

 character. When, for example, the proteins of Bact. tuberculosis are 

 injected into the circulation of susceptible animals, tubercle formation 

 occurs showing that these proteins are toxic. 



In certain infectious diseases also it is claimed by certain writers 

 that enzymes are responsible. This lacks substantiation. It is also 

 stated that in such infections, as anthrax, the mechanical effect of the 

 bacteria, plugging up the capillaries and producing mycotic emboli, 

 is a factor. This may be true but in addition other factors mentioned 

 above are concerned. 



In mixed infections of two or more microorganisms, which frequently 

 occurs, the infected individual may have within the body soluble toxins, 

 endotoxins and toxic bacterial proteins and in such a case it is difficult 

 to differentiate their action. 



THE METHODS BY WHICH INFECTIOUS MICROORGANISMS ARE 

 DISSEMINATED. 



The microorganisms of some of the infectious diseases, such as 

 diphtheria and Asiatic cholera and usually tetanus, remain local and 



