Fig 1.
Proposed physiological roles of Enterococcus faecalis polysaccharides in the gut.
(A) E. faecalis intestinal colonization has been proposed to be mediated by the formation of multicellular aggregates covered with and connected by a matrix (green) partly composed of exopolysaccharides [34,69]. (B) Formation of these complex communities may allow enterococci to evade the immune system by promoting survival within phagocytes (macrophages or neutrophils) and (C) to resist the effect of antibiotics produced by other gut bacteria. (D) Cells within these complex communities possess a variety of glycopolymers that promote aggregate formation and structural development [25]. Among these polymers, PG creates a layer that protects cells from osmotic pressure, reinforces cell shape and size, and shelters cell envelope components, including other structural/nonstructural polysaccharides [28]. The proposed scheme was based on previous publications [7,10,11] and shows an enterococcal cell envelope with relative glycobiological arrangements and positions. These glycopolymers, from left to right, include the membrane-anchored LTA (yellow), the exopolysaccharides containing polyGlcNAc (cyan), the covalently PG-bound CP (fuchsia), the WTA (light purple) that may also be bound to PG, and the EPA, which possesses a rhamnan backbone (dark purple) possibly anchored to PG and extracellular exposed WTA decorations (light purple). (E) WTAs, LTAs, EPA, and/or CPs allow E. faecalis to evade phagocytosis by macrophages or neutrophils [24,76,80,81,84–86]. These cell envelope components likely act as a shield that prevent complement detection and mediate resistance to phagocytosis. Changes to these polysaccharides may disrupt the cell envelope topography, leaving the bacteria naked or likely exposing “neoepitopes” for recognition by the complement system. (F) Changes in PG and EPA structure or composition may also be contributing factors in the resistance to antibiotics, such as those targeting the enterococcal cell envelope [13,17,20,24,53,55,57]. (G) E. faecalis may resist the intestinal bile salt toxicity through rearrangements in the composition of EPA and/or PG that counteract the detergent activity of these amphipathic molecules and the induced external osmotic pressure. (H) This bacterium can evade the immune response and persist in the gut by resisting the antimicrobial effect of lysozymes through modifications of their PG structure [81,95–97]. (I) EPA also acts as a “receptor” that is recognized by phages during viral infection, and changes in EPA decoration can confer resistance to these phages by E. faecalis [18,47]. (J) Upon intestinal overgrowth, this bacterium can exit the intestinal lumen to reach the bloodstream and colonize distal anatomical sites. The formation of matrix-covered enterococcal aggregates may represent a new mechanism that promotes E. faecalis translocation across the gut epithelial barriers in susceptible hosts, where EPA or/and polyGlcNAc-containing exopolymers may facilitate this migration. (?) denotes that the exact localization of EPA, polyGlcNAc-containing polysaccharides, and WTA in the enterococcal cell envelope is uncertain. CP, capsule polysaccharide; EPA, enterococcal polysaccharide antigen; LTA, lipoteichoic acid; PG, peptidoglycan; polyGlcNAc, β-(1,6)-GlcNAc polymers; WTA, wall teichoic acid.
Fig 2.
Comparison of predicted epa-like loci among enterococcal strains.
In Enterococcus faecalis V583 and OG1RF, EPA biosynthesis is encoded by a cluster of genes organized in 2 genetic loci: first, a conserved region, consisting of 18 genes, from epaA to epaR, which participate in the rhamnan backbone biosynthesis (core region), and, second, a downstream cluster of approximately 10–20 genes exhibiting genetic variability among strains, which has been proposed to account for the major differences on EPA decoration among E. faecalis isolates (variable region) [10,14,32,105]. The core region of Enterococcus faecium is differentially organized in comparison with the EPA core from E. faecalis. It does not have homologs of epaI, epaJ, and epaK. Instead, it has the 2 genes, epaP and epaQ, located at that site [104,105,107]. E. faecium’s variable locus is proposed to be divided into 4 main variants based on sequence similarities [104,107]. The scheme depicts variants 2 and 4 for strains 1,141,733 and Aus0004, respectively. Arrow colors correspond to colored boxes (bottom) and indicate predicted open reading frame function according to genome annotations and BLASTP analysis. Blue shades connect homologs of different strains. The absence/variability of epa genes suggests that the EPA polysaccharides of the 2 species have different sugar compositions, and, in consequence, may confer diverse physiological functions. The epa loci were adapted from [10,20,32,54,104,105,107]. All genes are drawn to scale. EPA, enterococcal polysaccharide antigen; GTF, glycosyltransferase.