Abstract
Enterococcus faecalis is a significant human pathogen worldwide and is responsible for severe nosocomial and community-acquired infections. Although enterococcal meningitis is rare, mortality is considerable, reaching 21 %. Nevertheless, the pathogenetic mechanisms of this infection remain poorly understood, even though the ability of E. faecalis to avoid or survive phagocytic attack in vivo may be very important during the infection process. We previously showed that the manganese-cofactored superoxide dismutase (MnSOD) SodA of E. faecalis was implicated in oxidative stress responses and, interestingly, in the survival within mouse peritoneal macrophages using an in vivo-in vitro infection model. In the present study, we investigated the role of MnSOD in the interaction of E. faecalis with microglia, the brain-resident macrophages. By using an in vitro infection model, murine microglial cells were challenged in parallel with the wild-type strain JH2-2 and its isogenic sodA deletion mutant. While both strains were phagocytosed by microglia efficiently and to a similar extent, the ΔsodA mutant was found to be significantly more susceptible to microglial killing than JH2-2, as assessed by the antimicrobial protection assay. In addition, a significantly higher percentage of acidic ΔsodA-containing phagosomes was found and these also underwent enhanced maturation as determined by the expression of endolysosomal markers. In conclusion, these results show that the MnSOD of E. faecalis contributes to survival of the bacterium in microglial cells by influencing their antimicrobial activity, and this could even be important for intracellular killing in neutrophils and thus for E. faecalis pathogenesis.
Highlights
Enterococcus faecalis is a natural member of the digestive microflora in humans and many other animals but in some instances it becomes a pathogen capable of causing severe human diseases (Gilmore et al, 2002), such as bacteraemia (Suppli et al, 2010), endocarditis (Fernandez Guerrero et al, 2007), neonatal sepsis (Sarkar et al, 2006) and meningitis (Pintado et al, 2003)
We previously showed that the manganese-cofactored superoxide dismutase (MnSOD) SodA of E. faecalis was implicated in oxidative stress responses and, interestingly, in the survival within mouse peritoneal macrophages using an in vivo–in vitro infection model
Similar results were obtained when different host : pathogen ratios were used in the assay. These findings suggest that MnSOD confers to E. faecalis resistance against the reactive oxygen species (ROS)-mediated killing and digestion by microglia
Summary
Enterococcus faecalis is a natural member of the digestive microflora in humans and many other animals but in some instances it becomes a pathogen capable of causing severe human diseases (Gilmore et al, 2002), such as bacteraemia (Suppli et al, 2010), endocarditis (Fernandez Guerrero et al, 2007), neonatal sepsis (Sarkar et al, 2006) and meningitis (Pintado et al, 2003). Enterococcal meningitis is rare, mortality is relatively high and occurs in 21 % of cases, posing additional challenges for the Abbreviations: DAPI, 4,6-diamidino-2-phenylindole; MnSOD, manganese-cofactored superoxide dismutase; ROS, reactive oxygen species. Microglia are brain macrophages that share many, if not all, the properties of macrophages in other tissues including the generation of free radicals, such as reactive oxygen species (ROS) during the respiratory burst, which is regarded as an important defence mechanism of the central nervous system against intracellular micro-organisms (Rock et al, 2004). E. faecalis has developed several enzymic and nonenzymic mechanisms to counteract deleterious effects of superoxide anion hydroxyl radical ((OOH22 ??)),, hydrogen peroxide which range from (H2O2) and DNA strand damage to peroxidation of membrane lipids (Imlay, 2003)
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