518 INFECTION A/YZ) IMMUNITY. 



virulence. Streptococci vary widely in their pathogenicity. 

 Cultures which are entirely non-pathogenic for 

 animals are frequently cultivated from nature and 

 from man. As a rule, however, the long chains 

 obtained from pathological processes in man are 

 pathogenic for rabbits and mice. Their virulence 

 is very labile, and by passage through suitable ani- 

 mals (rabbit, mouse) it may be pushed to a very 

 high point; in doing this, however, the original 

 virulence of the culture undergoes modifications. 

 For example, Marmorek so increased the virulence 

 of one strain that the millionth part of a cubic 

 centimeter was fatal for rabbits, but it had lost its 

 pathogenicity for man, as shown by inoculations 

 into carcinomatous patients. Hence the patho- 

 genicity of cultures for animals is not a good 

 index of their virulence for man. Those which 

 produce long chains in bouillon are more patho- 

 genic than those forming short chains (v. Lingel- 

 sheim). 



Eabbits and mice are the most susceptible ani- 

 mals. The rat, guinea-pig and cat, and larger ani- 

 mals, as the horse, goat and sheep, are less sus- 

 ceptible. A bouillon culture of which from 0.01 

 to 1.0 c.c. will kill a mouse or rabbit in from one 

 to four days is considered of high to moderate 

 virulence. Virulent cultures cause systemic infec- 

 tion, regardless of the method of inoculation. 

 Less virulent cultures produce changes which are 

 more localized in character and which may heal: 

 abscesses, areas of necrosis and erysipelatous 

 inflammations. 



The properties on which the virulence of strepto- 

 cocci depends are little understood. The conflict 

 of opinion concerning many points probably de- 



