210 BIOLOGY OF PNEUMOCOCCUS 



shown to exist when the intravenous route of infection is employed in 

 mice. 



The results of the studies by Enders and his colleagues were con- 

 temporaneously confirmed by the data obtained by Rich and Mc- 

 Kee 1138 in an investigation on native immunity of the rabbit to 

 Type III Pneumococcus. Strains of encapsulated Type III pneu- 

 mococci virulent for mice, when injected intradermally into rabbits 

 proliferate progressively in the tissues for some hours causing, 

 with the dose used, a local lesion of considerable severity, often ac- 

 companied by bacteriemia. The proliferation of the cocci is then 

 checked, the organisms are destroyed, and the animal completely 

 recovers. During the first twenty-four hours of the infective proc- 

 ess the rabbit acquires a greatly enhanced resistance to further in- 

 fection and the increased resistance is due to the fever developing 

 in the animal as a result of the infection. The encapsulated strains 

 to which the rabbit is resistant are not phagocyted promptly 

 either in vitro or in vivo, but after some hours' sojourn in the body 

 of the rabbit, the organisms gradually lose their capsules and are 

 avidly phagocyted. As in the body of the rabbit, the majority of 

 the strains studied exhibited marked sensitivity to temperatures of 

 104° to 106° in vitro. 



The work of Enders and his co-workers and of Rich and McKee, 

 therefore, offers a striking example of one type of mechanism op- 

 erating in a host-parasite relationship and again emphasizes the 

 dependence of virulence upon a special physiological instead of a 

 strictly immunological response on the part of a given animal 

 species. 



ARTIFICIAL, EXALTATION OF VIRULENCE 



There are several methods for raising the virulence of pneumo- 

 cocci. One is the frequent transplantation of the culture into a fa- 

 vorable medium, as advocated by Wadsworth and Kirkbride, 1471 

 and by Felton and Dougherty; 411 " 2 ' 423 " 4 another is serial passage 

 through suitable animals, or by inoculation of animals with an or- 



