ABSTRACT
The bacterial cell wall is a heteropolymer of glycan strands that are interconnected by the crosslinking of peptide stems found on alternate saccharides of the glycan strands. This cell wall is a key structural component of the cell envelope of most bacteria.260,511 The cell wall is an exoskeleton of the gram-positive bacteria, overlaying an inner (periplasm-like) space and above the single cell (plasma) membrane. The cell wall is an endoskeleton (found within the periplasmic space) of gram-negative bacteria. The cell wall is absent in the Mycoplasma within the phylum Mollicutes.140,477 Although the terms cell wall and cell envelope are often used interchangeably, the terms are distinct. The cell envelope is the multilayer barrier that separates and protects the cytoplasm of the bacterium from its external environment. The cell envelope includes the peptidoglycan cell wall, the membrane(s), the periplasmic space, and the macromolecules that are associated with these structures. The integrity of the cell wall is necessary for bacterial viability. The cell-wall heteropolymer is biosynthesized from both saccharide and amino acid precursors, and in its nal form constitutes a 3D polymer surrounding the bacterium. Hence, the cell wall in the mature bacterium is a single polymeric molecule-a sacculus-enveloping the bacterium. In recognition of the peptide stem cross-linking of the glycan strands, the term peptidoglycan is a synonym for the cell wall, particularly where the molecular basis of the cell-wall structure is emphasized. The term murein is an additional synonym for the cell-wall peptidoglycan. The glycan strands of the peptidoglycan consist of a repeating β-(1,4)-linked-[(N-acetylglucosamine)- (N-acetylmuramic acid)]–disaccharide, abbreviated by the acronym NAG-NAM. This disaccharide is highly conserved among both gram-negative and gram-positive bacteria, although some bacteria use conservative structural modications (such as O-acetylation) to impart resistance to autolysin degradation.70,479 The structure of the peptidic stems used for the cross-linking reaction is much more variable. Exhaustive degradation of the murein by glycosylases (such as lysozyme), lytic transglycosylases, and by stem amidases-the autolysin enzymes of the murein-gives an
Introduction .................................................................................................................................... 221 Gram-Positive and Gram-Negative Bacteria .................................................................................. 222 Molecular Structure of the Peptidoglycan ..................................................................................... 223 Peptidoglycan-Binding Proteins ....................................................................................................226 Peptidoglycan Biosynthesis ........................................................................................................... 227 Polymeric Structure of the Peptidoglycan .....................................................................................234 Peptidoglycan Growth, Remodeling, and Recycling ..................................................................... 236 Cell-Wall Biosynthesis Enzymes as Targets for Antibiotics .......................................................... 237 Peptidoglycan Integrity as a Response Stimulus ........................................................................... 238 Conclusion ..................................................................................................................................... 239 References ...................................................................................................................................... 239
array of constituent muropeptides. Muropeptide recognition is an important capability of the innate-immune system, and muropeptide excretion to subvert the innate-immune system is a virulence mechanism of certain bacterial pathogens.
