254 BACTERIOLOGY 



Thus far, then, infection must be viewed as a conflict 

 between bacteria on the one hand and tissues on the other; 

 the former having as their weapons of offence destructive 

 poisons; the latter, vital defensive provisions that enable 

 them to resist infection with greater or less degree of success, 

 according to circumstances. It makes no difference, there- 

 fore, whether, in infection, the bacteria be generally or only 

 locally present, the mechanism of infection is at bottom a 

 destructive intoxication. 



Bacterial Toxins. — ^The term "toxins," as used in bac- 

 teriology, refers to a group of soluable, nitrogenous, non- 

 crystallizable poisons that are elaborated by certain bacteria 

 in the course of their growth, both in the tissues of the living 

 host and under conditions of artificial cultivation. They 

 are assumed to be by-products of metabolism and they may 

 be separated easily from the living bacteria by which they 

 are manufactured by the simple process of filtration through 

 fine-pore earthenware filters. As they have not been ob- 

 tained in a pure state their chemical composition cannot be 

 stated precisely but it is probable that they are allied to the 

 globulins, nucleo-albumens, peptones, albumoses, or the 

 enzymes. 



The toxins are identified, not by their chemical structure, 

 but rather by their harmful action upon the tissues of living 

 animals, i. e., by their physiological reactions. It is this 

 property that renders them of such significance in the 

 phenomena designated as disease. 



By the injection of either of these bacteria-free, true 

 toxins into the tissues of susceptible animals, lesions are 

 produced that are in all essential respects identical with 

 those occurring in the course of infection by the living bac- 

 teria. By varying the dose of toxin injected into the animal 



