Infectious Agents. 57 



dal power ; and even thongh they have gained access into . the 

 blood, bacteria may by these agencies be rendered harmless. 



There must, therefore, be special conditions afforded before 

 a microorganism can become the actual cause of infection. In 

 the first place the inicrobe must possess peculiar toxic pozvers. 

 All microbes carry on metabolic processes and elaborate a va- 

 riety of chemical substances in the medium in which they exist. 

 If among these products there be any which are harmful to ani- 

 mal tissue, the possibility of toxic action exists. Since Brieger 

 called attention to these substances and indicated their relation to 

 disease occurrence, many of these microbic poisons have been 

 proved to exist by various investigators and the process of infec- 

 tion is recognized as invariably connnected with intoxication. 



In one class of microorganisms, the bacteria, poisons have 

 been demonstrated which are apparently free, soluble secretory 

 products of the bacterial cells, and which exhibit strong simi- 

 larity to ferments. These poisons are as yet not well defined 

 from a chemical standpoint; they are quickly rendered inert by 

 being heated (to 50° to 80° C.) and are peculiar in that they 

 do not manifest their activity at once, but require a certain 

 period of incubation. They are, moreover, especially peculiar 

 in their specificity and in the fact that zvJieii introduced into a 

 susceptible animal there is invariably a speciHc antitoxine gener- 

 ated. They are, therefore, not a homogeneous product of all 

 bacteria, but differ among themselves from their origin and the 

 particular kind of microbe, each as a special product of a special 

 process. These poisons are collectively called toxines. 



According to Ehrlich's theory, a toxine is a poison which possesses 

 two specific groups of atoms, one of which, the Iiaptophorc, anchors the 

 toxic molecule to the cellular protoplasm for which it has affinity ; the 

 second, or toxophorc group, doing the harm, supplying the real poisonous 

 agent. The toxines act then because they enter into chemical combination 

 with the cells. (Vide Chapter on immunity, pp. 18 and 22.) 



The proof that a microorganism secretes a toxine and through 

 it produces disease of the animal body is shown by the following : 

 Many bacteria can be grown artificially upon nutritive culture 

 media, as bouillon. If they secrete soluble toxines these will 

 accumulate in the medium. If such a culture be passed through 

 a filter impervious to bacteria, as a porcelain filter, the bacteria 

 will be retained by the filter and the bouillon will pass through 

 clear and quite free from bacteria. If there follow the injection 



