PATHOGENICITY FOR EXPERIMENTAL ANIMALS 205 



days, in a sample of medium otherwise suitable for maintenance of 

 virulence, resulted in a decrease in infective power of the pneumo- 

 cocci studied. 



According to Gaskell, 499 a medium made from beaten eggs was 

 satisfactory for preserving the virulence of pneumococcal cultures, 

 but the interpolation of fortnightly passages through the mouse 

 was necessary in order to restore waning infectivity. For addi- 

 tional methods for assuring or preserving the virulence of cul- 

 tures of Pneumococcus or of fluids or tissues containing the or- 

 ganism, the reader is referred to Chapter II. 



STRAIN VARIATIONS IN VIRULENCE 



The ability of a given strain of Pneumococcus to retain its viru- 

 lence over a long period and then suddenly to lose its invasive 

 power is too well known to require extended comment. Browning 

 and Gulbransen (1923) 161 described a culture of Type I Pneumo- 

 coccus, passed more than ninety times through mice during a pe- 

 riod of six years, which, when preserved in the dried spleen of an 

 infected mouse, exhibited marked variations in its ability to infect 

 mice. The conclusions reached by Gaskell (1928) 501 in a study of 

 the pathogenicity of single strains of pneumococci of Types I, II, 

 and III and Group IV may be cited. The virulence of Type II 

 strains was less than that of Type I organisms for both mice and 

 rabbits ; the pathogenicity of members of Type III was lower than 

 that of Type I for rabbits, mice, and also man ; whereas the viru- 

 lence of the Group IV cultures obtained from severe infections in 

 man was, if anything, higher than that of Type I pneumococci. 



Other individual strain differences with respect to the ability of 

 pneumococci to infect experimental animals have been reported by 

 Webster and Clow (1933). 1494 The degree of virulence of a strain 

 when inoculated into the nose of the mouse failed to parallel intra- 

 peritoneal virulence in 50 per cent of the strains studied — high 

 intranasal invasiveness being accompanied by either high or mod- 

 erate intraperitoneal virulence, and low intranasal by high, mod- 



