PATHOGENICITY FOR EXPERIMENTAL ANIMALS 211 



ganism of low virulence along with a killed culture of high viru- 

 lence ; while another method is to grow the organism in antirough 

 pneumococcic serum of the homologous type. Propagation in the 

 body of susceptible animals has long been practiced for the pur- 

 pose of enhancing the virulence of pneumococci. Eyre and Wash- 

 bourn (1898) 374 employed the guinea pig, Truche, Cramer, and 

 Cotoni (1911) 1426 the mouse, while Neufeld and Ungermann 

 (1912) 1001 " 2 increased the pathogenicity of Pneumococcus, when 

 administered to guinea pigs by inhalation, by passage through 

 animals of the same species. 



The procedure of serial animal passage is now too well known 

 to require extended comment. The intraperitoneal inoculation of 

 white mice with infectious material containing pneumococci or with 

 pure cultures, with subsequent recovery and cultivation of the or- 

 ganism from the heart's blood of the test animal is the method com- 

 monly used for raising and maintaining the virulence of pneumo- 

 cocci for experimental purposes or for the preparation of vaccines 

 for the immunization of horses in the production of therapeutic 

 serum as well as for testing the potency of such serums. The stimu- 

 lating action of the dead bodies of virulent strains of pneumococci 

 when injected simultaneously with living cultures of avirulent or- 

 ganisms of the corresponding serological type, resulting in the ac- 

 quisition of higher virulence, has been discussed in the chapter on 

 dissociation. A similar effect is produced by growth of organisms 

 of low virulence in media containing immune serum specific for de- 

 graded variants of the homologous type. 



DEGRADATION OF VIRULENCE 



Pneumococci as they encounter unfavorable conditions when cul- 

 tivated outside the animal body rapidly lose virulence. Insufficient 

 nourishment, excess of alkali or acid, unsuitable incubation or 

 storage temperatures, and infrequency of transplantation even in 

 a favorable medium, lower the vitality of the cell, inhibit capsule 

 formation, and consequently attenuate or destroy the pathogenic 



