A coagulase-positive strain of grown (strain O) was passaged in rabbits to increase virulence. The passaged organisms grown (strain P) were used to infect rabbits from which organisms grown (strain V) were isolated.

By LD50 measurement in rabbits, strain V was 25 times more virulent than strain P and 6000 times more virulent than strain O. On incubation in rabbit serum, strain V multiplied whilst strains P and O decreased in viable number (strain O the more rapidly) during the first 3 hr, recovering subsequently. In lysates of polymorphonuclear leucocytes, strain V multiplied or was slightly reduced in number; strains P and O decreased in number (strain O the more rapidly).

The findings suggest that resistance to serum and polymorph bactericidins is an important attribute of the virulence of staphylococci and that the substances responsible are qualitatively different or quantitatively increased on growth


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