PATHOGENICITY FOR EXPERIMENTAL ANIMALS 207 



for mice, while pneumococci that were invasive for guinea pigs 

 possessed high virulence for both rabbits and mice. In a second 

 paper, Truche and Cotoni 1424 described the effect of passing four 

 strains of Pneumococcus through mice, rabbits, and guinea pigs. 

 When strains virulent for mice were serially propagated through 

 animals of that species the organisms maintained their high viru- 

 lence but acquired no greater virulence over the original culture 

 for rabbits or guinea pigs. The same condition held true when 

 pneumococci were passed through rabbits, but there was no altera- 

 tion in the infectiousness of the strains for guinea pigs or mice. 

 Cultures subjected to passage through guinea pigs showed no in- 

 crease in pathogenicity for animals of that species or for mice or 

 rabbits. 



Twenty strains of Type III Pneumococcus studied by Levy- 

 Bruhl (1927) 804 were highly virulent for guinea pigs, moderately 

 so for mice, and very feebly or not at all virulent for rabbits. The 

 failure of many strains of Type III Pneumococcus to infect rab- 

 bits was also reported in the same year by Tillett. 1403 Of eleven 

 strains isolated from human sources, ten possessed low virulence 

 for rabbits despite the fact that all the strains possessed large 

 capsules and a high degree of virulence for mice. The odd strain 

 was rendered highly infectious for rabbits, and since it possessed 

 no other biological property demonstrably different from the other 

 ten strains, Tillett concluded that its individual virulence must re- 

 side in some additional property. 



The lack of virulence for rabbits of the majority of Type III 

 pneumococci has perplexed many bacteriologists, and no explana- 

 tion has been forthcoming for the disparity in virulence of Type III 

 strains until the recent communication of Enders and Shaffer. 860 

 Evidence was obtained that a correlation exists between the in- 

 ability of a strain to grow at 41° and virulence, since only among 

 the thermoresistant strains were found those possessing the prop- 

 erty of rabbit virulence. The attribute is constantly but not ex- 

 clusively associated with all Type III pneumococci, and was con- 



