INTRODUCTION 21 



at about the stage of the formation of peptone, and it has 

 long been known that peptones are quite highly poisonous 

 when administered parenterally. This also is' a crude 

 method of obtaining the poison, and with all the work that 

 has been done along this line, we do not know whether the 

 peptone is itself poisonous or whether its poisonous action 

 is due to admixture with some other digestive product. 

 We do know that as alimentary digestion proceeds the 

 protein poison itself is destroyed. Indeed, we had no 

 conception of the small amount of protein necessary to 

 furnish a lethal dose of the poison, until we submitted 

 proteins to the blood sera and organ extracts of sensitized 

 animals. Then we found that 1 mg. of protein may supply 

 enough poison to kill a guinea-pig when injected intra- 

 venously. But to produce the poison in this way necessi- 

 tates a delicate adjustment between substrate and ferment 

 which is imperfectly understood, and consequently inade- 

 quately controlled, and we can know that we have produced 

 the poison in any given experiment only by its effect on 

 an animal. Thus it happens that after years of study we 

 are still quite ignorant of the true nature and chemical 

 composition and structure of the protein poison. 



5. The pathogenicity of a bacterium is not determined by 

 its capability of forming a poison. Non-pathogenic bacteria 

 yield just as much of the protein poison as the most highly 

 pathogenic, and the proteins of our food contain the same 

 poison that is found in pathogenic bacteria. 



6. The pathogenicity of a bacterium is dependent upon its 

 ability to grow and multiply in the animal body. Any micro- 

 organism which can grow and multiply in an animal body 

 is pathogenic to that animal. Growing and multiplying 

 in the animal body means that the invader converts the 

 proteins of the animal into its own proteins, transforms 

 native into foreign proteins, and the accumulation of foreign 

 proteins can result only from the destruction of the native. 

 There are two conditions which determine whether or not a 

 foreign protein can grow and multiply in the animal body: 

 One is the capability of the invader of digesting and utilizing 



