Bacterial Cell Wall

Abstract

Most bacteria are encased in walls that protect the cells against lysis by osmotic forces from within and from chemical or biological assaults from outside. These walls are assembled in layers consisting of four principal components: inner membrane, peptidoglycan, outer membrane and S‐layer. The first two are the basic constituents of bacterial walls, the outer membrane is a hallmark of one branch of bacteria and S‐layers are optional in many different microorganisms. The two fundamental types of wall are Gram‐type positive, which have no outer membrane, and Gram‐type negative, which have an outer membrane. Bacteria also produce various secondary polymers, such as teichoic or mycolic acids, that associate with peptidoglycan and alter its chemical characteristics. Finally, and in addition to its protective role, the wall imparts to bacterial cells their specific shapes and is organised to facilitate the transport of vital chemicals into and out of the cell.

Key Concepts:

  • The cell wall protects bacteria from lysis, chemical assault and attack by the immune system.

  • The bacterial cell wall consists of an inner (plasma) membrane, a rigid peptidoglycan exoskeleton and, in some cases, an outer membrane and/or an S‐layer.

  • Peptidoglycan is composed of a long chain of repeating disaccharides linked to one another by short peptide side chains, which creates a single macromolecule surrounding the bacterial cell.

  • The biological properties of many bacterial cell walls are strengthened or enhanced by the addition of secondary cell wall glycopolymers.

  • The bacterial world is divided into two main groups: those that have a single membrane (the plasma or inner membrane) and those that have two membranes (inner and outer membranes).

  • S‐layers are paracrystalline arrays of a single protein that completely cover the exterior of many bacteria.

  • Bacterial shape is determined by cytoskeleton proteins that direct the synthesis of the overall structure of peptidoglycan.

Keywords: Gram‐positive walls; Gram‐negative walls; peptidoglycan; teichoic acid; periplasm; periplasmic space; S‐layers; cell shape; microscopy

Figure 1.

Basic types of bacterial cell walls. The individual components of the walls, which are layered on top of one another, are depicted as coloured rectangles: inner membrane (light blue), peptidoglycan (rust red), outer membrane (dark blue) and S‐layer (orange). Different types of walls are created by assembling various combinations of the components ((a)–(f)). Each type is described briefly underneath its column, and one or two examples of organisms that have that wall type are listed in parentheses.

Figure 2.

Electron micrographs of thin sections of bacterial cell walls. (a) The fibrous nature of the Gram‐positive cell wall of Bacillus subtilis. The fuzziness of the wall exterior is due to cell wall turnover. The wall is very thick but hydrolytic enzymes cut the peptidoglycan at intervals, beginning from the outside of the cell and working inwards, thus producing the hairy exterior seen here. Newly synthesised peptidoglycan is highly compacted and stains as a very dark band immediately above the (plasma) membrane. (b) The arrangement of a growing septum (arrows) in Bacillus licheniformis as the cell undergoes binary fission. During cell division of this Gram‐positive rod, the septum will grow inwards until the cell is bisected to divide the cell into two daughter cells. (c) Cross‐section of frozen‐hydrated B. subtilis 168. High magnification image of the envelope showing the plasma membrane (PM) enclosed by a low‐density inner wall zone (IWZ) which is bound by a high‐density outer wall zone (OWZ). Bar represents 50 nm. Note that this technique preserves and allows visualisation of a more complete structure than does the classical procedure in (a). Reprinted from Matias VRF and Beveridge TJ (2005). Molecular Microbiology56: 240–251, with permission from Wiley‐Blackwell. (d) A frozen‐hydrated section of the Gram‐negative cell wall from E. coli K12. The outer membrane (OM) and plasma (inner) membrane (PM) surround the cell. The space between the two is the periplasmic space. Within the periplasm is a thin line of peptidoglycan (PG) that is much less thick than the peptidoglycan of Gram‐positive cell walls. The bar equals 200 nm. Reprinted from Matias et al., with permission from the American Society for Microbiology.

Figure 3.

Schematic diagram showing the relationship between the glycan chains and peptide crosslinks in the most basic form of peptidoglycan.

Figure 4.

Schematic structures of selected cell wall glycopolymers (CWGs). Selected CWGs are divided into those attached to peptidoglycan and those anchored to the IM, with examples of individual organisms that contain each compound given in parentheses. The chemical constituents are denoted as follows: trioses (triangles), peptoses (pentagons), hexoses (hexagons), peptidoglycan sugar residues (teal), other sugars (yellow), sugar‐derived alcohols (orange), sugar‐derived acids (purple) and fatty acids (zigzag lines). Many structural details, such as sugar‐bonding patterns, are not included. It is thought most likely that N‐acetylglucosamine (GlcNAc) occurs in the first position of B. subtilisteichuronic acid and in the first position of the B. anthracis P‐CWG. Uncertain second positions are indicated by question marks. The exact position of pyruvylation in B. anthracis P‐CWG is unknown. Mycobacterial CWGs are more extensively branched than indicated in this schematic. Non‐glycosyl residues are designated as follows: A, d‐alanine; C, choline; M, mycolic acid; P, phosphate; S, succinate; Y, pyruvate. Glycosyl residues: AAT‐Gal, 2‐acetamido‐4‐amino‐2,4,6‐trideoxy‐d‐galactose; Ara, arabinose; Gal, galactose; GalNAc, N‐acetylgalactosamine; Glc, glucose; GlcA, glucuronic acid; Gro, glycerol; Ins, inositol; Man, mannose; ManNAc, N‐acetylmannosamine; Rha, rhamnose; Rto, ribitol. Reproduced from Weidenmaier and Peschel with permission from Nature Publishing Group.

Figure 5.

Electron micrograph of the negatively stained S‐layer from Synechococcus GL24. The subunits are hexagonally arranged and form a p6 lattice.

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Young, Kevin D(Apr 2010) Bacterial Cell Wall. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000297.pub2]