PROBLEMS OF PATHOGENIC MICRO-ORGANISMS 225 



Three possible fates await the invaders: (1) They are largely 

 destroyed within the body; (2) they are excreted, or discharged 

 through various channels; (3) they remain indefinitely in the 

 body after the disease is over, to be eventually destroyed or elimin- 

 ated. 



That the micro-organisms are largely destroyed within the body 

 in the course of the disease is not open to dispute; this class is of 

 no special significance to us. Of most importance are those that 

 escape to continue their life-cycle in another subject. The mechan- 

 ism of elimination is of vital importance to the parasite. It assumes 

 many forms, and is admirably adapted in the various specific diseases 

 to perpetuate the existence of the species. 



The survival of the microbes after the disease is over may be 

 explained partly on the ground that in nearly all diseases some of 

 the microbes pass their final stage near the surface of the skin, or 

 mucous membrane, or in organs in direct or in indirect contact with 

 the outer air, so that escape outward is readily effected through 

 destruction of tissue, and hence protection from the bactericidal 

 forces of living tissue. The small number which in some types of 

 disease remain alive for some time after the disease process has 

 subsided may also be inclosed in small foci of necrotic tissue, and 

 thus escape destruction temporarily. 



I am inclined to belie ve, however, that among the problems of 

 the future will be the elucidation of still another mechanism for the 

 protection and escape of the micro-organism. It is highly probable 

 that in a certain number of species of bacteria, after the active 

 vegetative stage a latent stage follows, during which the parasite 

 which has escaped destruction provides itself with some protective 

 envelope which also aids it in its passage to a new host. This envelope, 

 which may be some specific substance not recognizable with the 

 microscope, or which may be represented by the capsules in some 

 groups, may be a defensive body of the parasite stimulated to over- 

 production by the anti-bodies of the host. This body also interferes 

 with the metabolism of the microbe, and thus acts in the double 

 capacity of stopping the disease and protecting the microbe at the 

 same time. This hypothesis suggested itself to me while endeavoring 

 to account for the peculiar behavior of tubercle bacilli under culti- 

 vation. 



It is well known that tubercle bacilli from the diseased tissues 

 of cattle grow very slowly, and then only upon certain culture- 

 mediums, such as blood-serum. After several years of continuous 

 cultivation they multiply vigorously in glycerin bouillon, and can 

 hardly be distinguished in appearance from those human varieties 

 of the bacillus which grow richly from the first or second transfer. 

 There seemed to be no justification to assume that the bacillus had 



