Evasion of host innate immunity by Enterococcus faecalis: the roles of capsule and gelatinase

Abstract

Enterococci are gram-positive bacteria typically found as commensals in the gastro-intestinal tracts of most mammals. Enterococci, most notably Enterococcus faecalis and Enterococcus faecium, have become problematic causative agents of several nosocomially acquired infections including urinary tract infections, bacteremia, surgical sight infections, and endocarditis. These bacteria must first overcome the innate immune response in order to establish infection. Many bacteria produce capsular polysaccharides that contribute to pathogenesis by helping the microbe evade the host innate immune response. The capsular polysaccharide produced by E. faecalis has been shown to play a role in pathogenesis; however the mechanisms of innate immune avoidance were unknown. Moreover, the number of capsule serotypes produced by E. faecalis and the genetic differences that contribute to capsule serospecificity were in doubt. In the current study it is made clear that only two capsule serotypes are produced by E. faecalis and that both capsule serotypes contribute to evasion of the host innate immune system. This work shows two mechanisms by which the capsule of E. faecalis contributes to immune evasion. First, the presence of capsule inhibited complement mediated phagocytosis through limiting the detection of opsonic complement protein C3 on the surface of the bacteria. Secondly, the presence of capsule altered cytokine signaling of macrophages by shielding bacterial components from detection. Many pathogenic strains of E. faecalis also produce an extracellular protease known as gelatinase (GelE). This work also shows a novel mechanism involving GelE in innate immune evasion through the degradation of the anaphylatoxin C5a. Degradation of C5a by GelE resulted in decreased neutrophil recruitment in vitro. A rabbit model of endocarditis was employed to assess the effect of GelE production on disease development and progression. Rabbits infected with GelE producing strains had increased bacterial burdens in the heart compared to rabbits infected with strains that were GelE negative. Reduced phagocyte infiltration at primary and secondary infection sites was also observed in rabbits infected with GelE producing strains compared to GelE negative strains. The work presented here demonstrates that both the capsular polysaccharide and GelE play roles in E. faecalis evasion of innate immune responses. Moreover, these pathogenic determinants would be suitable targets for developing alternative therapeutics used to treat E. faecalis infections.