THE AVENUE OF INFECTION AND TISSUE SUSCEPTIBILITY 97 



indeed with the various organs of the same animals, it is usual to find 

 that virulence raised by animal passage affects only the animal or the 

 particular organ of a certain animal, and not all animals in general. 

 Thus, in general, the passage of bacteria through rabbits increases their 

 virulence for rabbits and not for mice, dogs, pigeons, etc.; passage 

 through mice may increase their virulence for mice, but not for rabbits, 

 guinea-pigs, etc. 



(b) The use of collodion sacs for increasing virulence has been ad- 

 vocated, especially by French investigators. When microorganisms 

 are inclosed in a collodion capsule of the proper thickness and placed 

 within the abdominal cavity of a suitable animal, the slightly modified 

 body-juices are able to transfuse through the sac, impeding the develop- 

 ment of such microorganisms as are unable to immunize themselves or 

 withstand the injurious influences. In this manner a race of virulent 

 bacteria are artificially selected which can endure the defensive agencies 

 of those juices with which they have come into contact. 



(c) The addition of animal fluids to the culture-medium may enable 

 the bacteriologist to maintain or even to increase the virulence of a 

 microorganism according to the principles of artificial selection. The 

 fluid, either a serum or whole blood, is secured in a sterile manner and 

 added to the medium in a raw or unheated condition. In this manner 

 the microorganisms are exposed to some of the defensive agencies con- 

 tained in the juices under these conditions, and this tends to destroy 

 the less resistant bacteria, encourage the more resistant, and at least 

 maintain, for a longer or a shorter time, the virulence of a culture 

 freshly isolated from a lesion or cultivated by animal passage. 



THE AVENUE OF INFECTION AND TISSUE SUSCEPTIBILITY 



Successful infection of the body by certain bacteria can be accom- 

 plished only when invasion takes place through appropriate avenues. 

 Thus typhoid, cholera, and dysentery infection seems to take place 

 through the gastro-intestinal tract, and doubtfully by inhalation, and 

 not at all through the skin or urogenital system; gonococci usually 

 enter the body through the genital organs or the eye, and not through 

 the respiratory apparatus or through the skin. The route of infection 

 is less important with microorganisms characterized by great aggres- 

 siveness and producing general, rather than local, infections. For 

 example, in most animals anthrax is a general bacteremia, regardless 

 of the route of invasion; plague rapidly becomes a bacteremia, whether 

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