THE INFECTIVE PROCESS 147 



(4) Poisons which are typically intra-cellular, but which 

 may also be excretory. The poisons produced by the 

 typhoid bacillus, the Bacillus coli, the Bacillus enteritidis 

 of Gaertner, and the cholera vibrio belong to this group. 



Thiele and Embleton 1 suggest that the toxins of bacteria 

 are really cleavage products derived from their cellular 

 proteins under the influence of ferments present in the 

 body of the host. These cleavage products are, however, 

 toxic only at a certain stage of their disintegration. Given 

 the power of existing and multiplying in the body of the 

 host, the pathogenicity of a bacterium depends on the 

 quantity and consequent activity of the ferments of the 

 host. A certain degree of ferment activity renders the 

 cleavage products of the bacterio-protein toxic, a further 

 degree of ferment activity carries the disintegration so far 

 that the cleavage products are no longer toxic. A 

 bacterium may therefore be harmless to a host if the 

 latter (a) has no ferments capable of digesting its bacterio- 

 protein ; (b) has such a poor supply of ferments that the 

 bacterio-protein is so slowly disintegrated that toxic 

 products never attain a sufficient concentration to be 

 harmful ; (c) has such a plentiful supply of ferments that 

 the cleavage of the bacterio-protein rapidly passes beyond 

 the toxic stage. A harmless bacterium, e.g. B. megaterium> 

 may be rendered pathogenic if suitable ferments can be 

 produced in the host to bring about the necessary dis- 

 integration of its bacterio-protein. 



The Infective Process 



With regard to the pathogenic micro-organisms, or 



disease germs, Koch laid down the following conditions, 



which have been termed " Koch's postulates," which 



must be complied with before the relation of an organism 



1 Lancet, vol. i, 1913, pp. 234 and 332. 



