INFECTION AND INFECTIOUS AGENTS 33 



body and for which Behring and Kitasato (1890) have shown that 

 antitoxins can be developed. Pathogenic streptococci also produce 

 a substance, leucocidin, toxic for leucocytes. McLeod (1914) and 

 Gay (1931) regard this as an important factor in the pathogenicity 

 of streptococci. It is now generally agreed that each type of 

 bacteria owes its specificity to its chemical constitution and in some 

 cases to specific chemical products secreted by it. All diphtheria 

 bacilli are not only more or less alike chemically and also chemically 

 different from other bacteria, but they likewise all secrete a specific 

 poison called diphtheria toxin. The true toxins differ from other 

 poisons in that antitoxins are formed by the body in response to 

 their presence within tlie tissues. One can then define a true toxin 

 as an antigenic poison. 



It has been customary to classify bacterial poisons into "exo- 

 toxins" and "endotoxins." The former are highly antigenic and 

 are readily liberated from the bacterial cell while the "endotoxins" 

 either fail to stimulate specific neutralizing substance or at best 

 produce only a feeble response and are liberated with more or 

 less difficulty from the bacterial cell. Eaton (1939) states, in effect, 

 that a sharp line cannot be drawn between ' ' exotoxins ' ' and ' ' endo- 

 toxins" because antigenic poisons exhibit all degrees of anti- 

 genicity and toxicity and also marked differences in the ease with 

 which they can be separated from the bacterial cells. For a more 

 extensive discussion of the subject the student is referred to 

 Eaton's (1939) excellent review of chemical investigations of 

 ])acterial toxins and to the discussion of toxins by Zinsser, Enders, 

 and Fothergill (1939). 



Botulism. — In botulism the disease is due to a toxin formed 

 ])y the bacteria outside the body, taken in with the food and ab- 

 sorbed through the mucous membrane of the intestine. 



Tetanus. — In tetanus the organisms gain entrance to a wound 

 or are mechanically carried into the tissues by a foreign body. If 

 anaerobic and other satisfactory conditions prevail, the bacteria 

 grow saprophytically and secrete tetanus toxin. This, according 

 to Abels (1934), is carried to the central nervous system by the 

 blood stream. The evidence, however, seems to indicate that the 

 toxin is absorbed, probably through the end plates of the motor 

 nerves, and reaches the central nervous system along their axis 

 cylinders. When the toxin reaches the central nervous system, 

 there develop certain physiological disturbances called tetanus. 



