Effect of immunization on the genesis of pneumococcal endocarditis in rabbits

The effect of immunization with whole organisms on the development of Streptococcus pneumoniae endocarditis was examined by in vivo and in vitro methods. Immunization protected rabbits from pneumococcal endocarditis when the in vivo catheterization model was used. The inoculum size that caused endocarditis in 50% of the unimmunized rabbits was 1.1 X 10(5) colony-forming units, whereas 1.2 X 10(7) colony-forming units were required for infecting 50% of the immunized rabbits (P less than 0.001). Investigations were carried out to determine the mechanism which enabled immunization to prevent the development of pneumococcal endocarditis; they indicated that a reduction in bacterial adherence could not explain this phenomenon. In vitro studies showed that subagglutinating quantities of antibody increased the adherence (P less than 0.05) of pneumococci to rabbit aortic valve cusps. The adherence ratio of pneumococci to fibrin-platelet clots was at least doubled by the presence of subagglutinating dilutions of immune sera (P less than 0.001). Further studies showed that immunoglobulin G in the immune sera accounted for this increased in vitro adherence. However, further in vivo studies showed that immunized rabbits were able to clear live pneumococci from their bloodstreams within 4.5 h, whereas unimmunized rabbits failed to clear the organism within 24 h.