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Category: Bacterial Pathogenesis
Enterococcal Cell Wall, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555817923/9781555812348_Chap05-1.gif /docserver/preview/fulltext/10.1128/9781555817923/9781555812348_Chap05-2.gifAbstract:
Walls of the enterococci may represent 27 to 38% of the dry cell weight (exponential and stationary phase cells, respectively). Three main constituents are generally reported: peptidoglycan (PG), teichoic acid, and polysaccharide. Sometimes, proteins are also mentioned. Most of the ultrastructural analyses of the enterococcal cell walls were conducted in Enterococcus hirae ATCC9790. Structure, biosynthesis, and assembly of the different polymers that constitute the enterococcal cell walls are discussed in this chapter. The backbone of the enterococcal wall is PG, which is organized as a fisherman's net. One of the major functions of the PG in gram-positive organisms is the resistance to bursting induced by high cytoplasmic osmotic pressures. Many details of cell wall synthesis by enterococci are known, thanks in large measure to the extensive study of the mechanisms underlying vancomycin resistance. Cell wall-associated proteins have most extensively been studied in staphylococci and streptococci. Three categories of surface proteins are usually distinguished: those that have a LPXTG motif and anchor at their C-terminal ends, those that bind by way of charge and/or hydrophobic interactions, and those that bind by their N-terminal end. Few proteins have been described that bind through charge and/or hydrophobic interactions. Enterococcus faecalis and Enterococcus faecium cells can exchange genetic material (plasmids) by conjugation processes induced by small peptide pheromones. Analysis of the gene sequences of E. faecalis indicated that the pheromones produced by plasmid-free strains originate from the signal sequences of apparent lipoprotein precursors.
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Model of the cell wall of a gram-positive organism. The multilayered peptidoglycan covers the cytoplasmic membrane bearing embedded proteins and lipoteichoic acids. To the peptidoglycan are bound or associated teichoic acids (rods), polysaccharides (hexagons), and proteins (small and large spheres). Modified from reference 35 with permission.
Model of the cell wall of a gram-positive organism. The multilayered peptidoglycan covers the cytoplasmic membrane bearing embedded proteins and lipoteichoic acids. To the peptidoglycan are bound or associated teichoic acids (rods), polysaccharides (hexagons), and proteins (small and large spheres). Modified from reference 35 with permission.
Cross-linked peptidoglycan of enterococci. The A3α type with an (Ala)2-3 cross-bridge is found in E. faecalis. The A4 α type with a d-isoAsn cross-bridge is found in E. faecium, E. hirae, and several other species.
Cross-linked peptidoglycan of enterococci. The A3α type with an (Ala)2-3 cross-bridge is found in E. faecalis. The A4 α type with a d-isoAsn cross-bridge is found in E. faecium, E. hirae, and several other species.
Biosynthesis of the peptidoglycan of E. faecium. The genes in bold indicate those that were identified in enterococci (see text).
Biosynthesis of the peptidoglycan of E. faecium. The genes in bold indicate those that were identified in enterococci (see text).
Model of the cell wall surface enlargement of E. hirae. The equatorial wall band marks the site of wall synthesis. It is first notched at the same time as the nascent septum starts to grow down. The septum elongates and is concomitantly split apart to make a new (clear) peripheral wall. Finally, the septum closes up the central gap and divides the original cell in two compartments. Reproduced from reference 4 with permission.
Model of the cell wall surface enlargement of E. hirae. The equatorial wall band marks the site of wall synthesis. It is first notched at the same time as the nascent septum starts to grow down. The septum elongates and is concomitantly split apart to make a new (clear) peripheral wall. Finally, the septum closes up the central gap and divides the original cell in two compartments. Reproduced from reference 4 with permission.
Proposed model of peptidoglycan assembly by a double channeling of cell wall precursors. Channel A is primarily involved with the synthesis of new cross wall. Channel A is involved in the conversion of the cross wall into two layers of thickening peripheral wall. Reproduced from reference 74 with permission.
Proposed model of peptidoglycan assembly by a double channeling of cell wall precursors. Channel A is primarily involved with the synthesis of new cross wall. Channel A is involved in the conversion of the cross wall into two layers of thickening peripheral wall. Reproduced from reference 74 with permission.
Variation in cell wall carbohydrate polymer species from the cell wall of E. faecalis strain FA2-2. Polysaccharides were separated by electrophoresis through 10% polyacrylamide and detected with Stains-all. Lane 1, total cell wall polysaccharides; lane 2, capsular polysaccharide; lane 3, purified enterococcal polysaccharide antigen; lane 4, purified cell wall teichoic acid. The mean molecular size (kDa) of each carbohydrate, as assessed by gel filtration, is shown at left.
Variation in cell wall carbohydrate polymer species from the cell wall of E. faecalis strain FA2-2. Polysaccharides were separated by electrophoresis through 10% polyacrylamide and detected with Stains-all. Lane 1, total cell wall polysaccharides; lane 2, capsular polysaccharide; lane 3, purified enterococcal polysaccharide antigen; lane 4, purified cell wall teichoic acid. The mean molecular size (kDa) of each carbohydrate, as assessed by gel filtration, is shown at left.
Proposed pathway for the synthesis of the serotype capsular polysaccharide in E. faecalis. In the first step of polysaccharide biosynthesis, the monosaccharides (represented by the small circles) must be activated by linkage to a nucleotide diphosphate (displayed as a circle with an asterisk). The nucleotide diphosphate provides the necessary energy to catalyze the polymerization of the monosaccharides into the oligosaccharide subunit by the glycosyl transferases shown in step 2. As a third step in the biosynthesis of the polysaccharide, the membrane-bound ABC transport proteins shuttle the oligosaccharide subunit across the cell membrane to the cell wall, where the polysaccharide is anchored to the cell. The transport of additional oligosaccharide subunits allows polymerization into a growing polysaccharide chain.
Proposed pathway for the synthesis of the serotype capsular polysaccharide in E. faecalis. In the first step of polysaccharide biosynthesis, the monosaccharides (represented by the small circles) must be activated by linkage to a nucleotide diphosphate (displayed as a circle with an asterisk). The nucleotide diphosphate provides the necessary energy to catalyze the polymerization of the monosaccharides into the oligosaccharide subunit by the glycosyl transferases shown in step 2. As a third step in the biosynthesis of the polysaccharide, the membrane-bound ABC transport proteins shuttle the oligosaccharide subunit across the cell membrane to the cell wall, where the polysaccharide is anchored to the cell. The transport of additional oligosaccharide subunits allows polymerization into a growing polysaccharide chain.
Model for the organization of cell wall polymers in the cell wall of E. faecalis. The lipid-anchored lipoteichoic acid, also known as the streptococcal group D antigen, is shown protruding into the cell wall peptidoglycan. Shown anchored to N-acetylmuramic acid (MNAc) residues of the peptidoglycan are the integral cell wall teichoic acids and the hypothesized enterococcal species antigen. Anchored to the N-acetylglucosamine (GNAc) residues in the peptidoglycan and protruding out from the peptidoglycan is the serotype-specific capsular polysaccharide.
Model for the organization of cell wall polymers in the cell wall of E. faecalis. The lipid-anchored lipoteichoic acid, also known as the streptococcal group D antigen, is shown protruding into the cell wall peptidoglycan. Shown anchored to N-acetylmuramic acid (MNAc) residues of the peptidoglycan are the integral cell wall teichoic acids and the hypothesized enterococcal species antigen. Anchored to the N-acetylglucosamine (GNAc) residues in the peptidoglycan and protruding out from the peptidoglycan is the serotype-specific capsular polysaccharide.