PATHOGENICITY FOR EXPERIMENTAL ANIMALS 213 



ity or resistance depends on peculiarities due to the species, on 

 genetic factors, age, weight, environmental conditions, and the 

 physical state of the test animal. 



In addition to the variables in the host, the invasiveness of a 

 given strain of Pneumococcus is conditioned by its serological 

 type, the vital condition of the culture, its mass or density, the 

 route of inoculation, and by intrinsic factors possessed by various 

 strains of the organism. Virulence of a pneumococcus for a given 

 animal species may be raised by serial passage through animals of 

 the same species, and the enhanced pathogenicity can be main- 

 tained by continued animal passage or by the application of suit- 

 able in vitro methods of preservation. Contrariwise, the pathogen- 

 icity of a pneumococcal strain may be decreased by subjecting the 

 organisms to unfavorable cultural conditions or by propagating 

 the organisms in media containing increasing amounts of homolo- 

 gous immune serum. Pathogenicity or virulence, therefore, is only 

 a relative term and must be interpreted in the light of the biologi- 

 cal characters of the pneumococcal strain and of the functional 

 variables in the animal host. As yet no in vitro test has been de- 

 vised which equals the use of animals, particularly the mouse and 

 the rabbit, for studying the invasive power or virulence of Pneu- 

 mococcus. 



