84 POISONOUS PROTEINS 



former must be able to digest the proteins of 

 the animal. In the second place the invading 

 cells must not be immediately destroyed by the 

 ferments elaborated by the body cells. There 

 must be a supporting relation between the bac- 

 terial cell and the medium, and in infection the 

 body constitutes the medium in which the bac- 

 teria grow and multiply. The protein groups 

 split from the medium must fit into the molecu- 

 lar structure of the bacterial cell; otherwise 

 they would be of no service to it. Many kinds 

 of cells may live in the same medium, but for 

 each kind the cleavage, of the medium must be 

 specific. From this it follows that the agent by 

 which the cleavage products are secured must 

 be supplied by the cell and must be specific to it. 

 It follows from what has been said that a 

 bacterium placed in a medium in which its fer- 

 ment is ineffective cannot grow and multiply. 

 A bacterium which cannot grow and multiply 

 in the animal body cannot cause an infection. 

 Its inability to grow and multiply in the animal 

 body may be due to the fact that its ferments 

 cannot digest or properly break up the proteins 

 of the animal body. This is one of the reasons 

 why the great majority of bacteria are harm- 

 less or nonpathogenic. This, however, is not 

 the sole, and probably not the dominant cause 



