STREPTOCOCCUS PYOGENES 



345 



experimental enhancement or reduction of the virulence. Streptococcus 

 hemolysins may be conveniently observed by cultivation of the organ- 

 isms upon blood-agar plates. They may be produced in alkaline pepton- 

 broth and obtained separate from the bacteria by nitration a procedure, 

 however, in which the quantities obtained are never large. Besredka 1 

 and Schlesinger 2 believe, for this 

 reason, that the hemolytic sub- 

 stances are closely attached to the 

 bacterial bodies. The last-named 

 author, furthermore, has deter- 

 mined that, in contradistinction to 

 the other toxic substances, strepto- 

 coccus hemolysins are extremely 

 labile, disappearing from culture 

 fluids after standing for from five 

 to seven days at ordinary room 

 temperature. 



Immunization. For reasons not 

 wholly understood at present, re- 

 covery from streptococcus infection 

 does not to any marked degree 

 produce immunity against these 

 bacteria. Active immunity may, 



however, be produced in rabbits, goats, horses, and other domestic 

 animals by treatment with gradually increasing doses of streptococcus 

 cultures. 3 



In carrying out such immunizations it is necessary to use for the first 

 injection attenuated or dead bacteria. Attenuation may be accom- 

 plished by moderate heating or by the addition of chemicals (terchloride 

 of iodin). Neuf eld 4 advises, for the first injection in immunizing 

 rabbits, the use of ascitic-broth cultures killed by heating to 70 C. This 

 is followed, after ten days, by a second injection of a small quantity of 

 fully virulent cocci. Following this, injections are made at intervals 

 of ten days with constantly increasing doses. Modifications of. these 

 general principles are employed in most laboratories. 



The sera of animals so treated contain no demonstrable antitoxic or 



1 Besredka, Ann. de 1'inst. Pasteur, xv, 1901, p. 880. 



2 Schlesinger, Zeit. f. Hyg., xxiv, 1903. 



FIG. 74. STREPTOCOCCUS COLONIES 

 FROM BLOOD CULTURE ON BLOOD- 

 AGAR PLATE. Showing areas of hemol- 

 ysis about colonies. 



3 Koch und Petruschky, Zeit. f. Hyg., xxiii, 1896. 

 *Neufeld, Zeit. f. Hyg., xliv, 1903. 



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