Prophylactic and therapeutic efficacy of antibodies to a capsular polysaccharide shared among vancomycin-sensitive and -resistant enterococci

Enterococci are important nosocomial pathogens that are increasingly diffic ult to treat due to intrinsic and acquired resistance to antibiotics, inclu ding vancomycin, A recently described capsular polysaccharide (CP) isolated from Enterococcus faecalis 12030 was used to evaluate the potential effica cy of active or passive immunotherapy regimens as adjunctive treatments. Ev aluation of protective efficacy was carried out in immunocompetent mice cha llenged intravenously (i.v.) with live enterococci. In nonimmune mice, i.v. inoculations resulted in high levels of bacteria in kidneys, spleens, and livers 5 days after challenge. Mice immunized with four 10-mu g doses of CP antigen/mouse were protected against challenge with the homologous E. faec alis strain. High-titer opsonic immunoglobulin G was also induced by immuni zing rabbits with the purified CP, and passive transfer of this antiserum t o mice produced significantly lower bacterial counts in organs than did nor mal rabbit serum or sterile saline. Antibodies to the polysaccharide isolat ed from E. faecalis 12030 were protective against Enterococcus faecalis OG1 RF and against two serologically related, vancomycin-resistant Enterococcus faecium clinical isolates. Antibodies to this CP antigen were also effecti ve as a therapeutic reagent in mice when passive therapy was initiated 48 h after live bacterial challenge. These data indicate that CP antigens from enterococci are potential targets of protective antibodies and that these a ntibodies may be useful for prophylaxis and treatment of enterococcal infec tions.