OBSTACLES TO INFECTION 27 



occlusion of hair follicles and the ducts of sweat glands by little plugs 

 of bacteria constitutes an injury just as well as a visible abrasion of 

 the surface. Such little plugs of bacteria may, on the one hand, 

 act as foreign bodies and mechanically damage the more delicate 

 structures with which they come in contact, or the bacteria as such, 

 through their own secretory or degenerative products may cause a 

 local destruction of these more delicate structures and thus open a 

 route to infection of the underlying tissues. This possibility must 

 also be considered in cases where infection takes place apparently 

 directly from the very surface of epithelial linings, but is naturally 

 less likely to play a role in so dense a structure as the surface epi- 

 thelium of the skin, as in the case of the mucous membranes. 



The infection of the urethral mucosa by the gonococcus is fre- /^.^ 

 quently cited as an example of the possibility of infection through 

 intact epithelium. But we know that filtrates of gonococcus cultures 

 are in themselves capable of producing a mucopurulent inflammation 

 of the human urethral mucous membrane, so that here again no 

 proof has been afforded that the organism can infect through intact 

 epithelium. 



That toxin production, on the other hand, does not necessarily lead 

 to infection is well shown in the case of the diphtheria bacillus. We 

 know 7 that the toxin production of this organism is fairly constant and 

 that there is no ground for believing that toxin is not formed by 

 those bacilli which may at times be found in the throats of perfectly 

 healthy individuals. As the amount of toxin is, however, evidently 

 not sufficient to cause local necrosis, we must assume that conditions 

 for active multiplication of the organism are not favorable, and we 

 can readily believe that a break in the continuity of the epithelium 

 at some point might be the essential factor which would lead to an 

 actual infection. This break may occur through mechanical means, 

 but it may also occur through the intervention of associated patho- 

 genic agents, and I would emphasize particularly the importance 

 of underlying pyogenic infections, for the occurrence of which the 

 ground is especially favorable in the lymphadenoid structures of the 

 fauces and the nasopharynx. 



The importance of such associated infections cannot be over- 

 estimated. We have good evidence to show that in their absence, 

 infection with certain bacteria cannot occur at all. It is thus well 

 known that the tetanus bacillus cannot maintain itself, when in- 



