438 PROTEIN POISONS 



structure of the cell, otherwise they would be of no service 

 to it. This necessitates the cleavage of the molecules of 

 the medium along definite lines. Many kinds of cells may 

 live in the same or like media, but for each kind of cell 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 itself, and must be 

 peculiar to that kind of cell. These cleavage agents which 

 prepare foods for the cell from the medium are known as 

 ferments, and each kind of cell has its own characteristic 

 and specific ferments. As to the real nature of ferments, 

 we know little or nothing, but that every kind of cell has 

 its specific ferment or ferments, we do know. The same 

 ferment may not be able to break up all proteins. In this 

 respect there are great variations in the proteolytic fer- 

 ments. Some digest a wide variety of proteins while others 

 are capable of acting only on one specific protein. There 

 must be a relation between the ferment and its substrate. 

 As Fischer once said, the former must fit the latter as a 

 key fits into the lock, and as there are master keys that 

 open many doors, so there are general proteolytic ferments, 

 and as there are special keys that fit only one lock, so there 

 are specific proteolytic ferments. It will be observed that 

 we have used the word " specific" in two senses in speaking 

 of proteolytic ferments. Each kind of cell has its specific 

 ferment, and each protein may have its specific ferment. 

 This double use of the term "specific" should be borne in 

 mind, since there seems to be no way to avoid it. 



It follows from what has been said that a bacterium 

 placed in a medium in which its ferment 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 ferment or 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 non-pathogenic or are harmless. These organisms 

 when grown on suitable media produce just as much poison 



