Virulence 297 



Virulence. Pneumococci vary greatly in virulence, and 

 rapidly lose this quality in artificial culture. When it is 

 desired to maintain or increase the virulence, a culture must 

 be frequently passed through animals. Washbourn found, 

 however, that a pneumococcus isolated from pneumonic 

 sputum and passed through one mouse and nine rabbits 

 developed a permanent virulence when kept on agar-agar 

 so made that it was not heated beyond 100 C., and alka- 

 linized 4 c.c. of normal caustic soda solution to each liter 

 beyond the neutral point determined with rosolic acid. 

 The agar-agar is first streaked with sterile rabbit's blood, 

 then inoculated. The cultures are kept at 37.5 C. Ordi- 

 narily pneumococci seem unable to accommodate them- 

 selves to a purely saprophytic life, and unless continually 

 transplanted to new media die in a week or two, some- 

 times sooner. Lambert found, however, that in Marmorek's 



Fig. 86. Diplococcus pneumoniae. Colony twenty-four hours old upon 

 gelatin. X 100 (Frankel and Pfeiffer). 



mixture of bouillon 2 parts and ascitic or pleuritic fluid 

 i part, the organisms would sometimes remain alive as long 

 as eight months, preserving their virulence during the entire 

 time. 



Virulence can also be retained for a considerable time by 

 keeping the organisms in the blood from an infected rabbit, 

 hermetically sealed in a glass tube, on ice. 



