Cell Wall in Various Bacteria’s | Microbiology

In this article we will discuss about the chemical compositions of cell wall in various bacteria’s.

The cell wall of bacteria is made up of network of peptidoglycan (murein, mums means wall). It is present almost on all bacterial cell wall except Halo-bacterium and Halo-coccus. Because these bacteria live in marine water which contains high salt concen­tration. The osmotic pressure of cytoplasm is more or less similar to outside the cell environment.

Peptidoglycan determines the shape of a cell. It accounts for 40-80% of total dry weight of cell. Its thickness is about 30-80 mn. It is insoluble and porous polymer that provides rigidity. It is a mucopolysaccharide. However, its chemical composition differs from species to species.

It consists of repeating disaccharides attached to chains of four or five amino acids. The monosaccharides, N-acetyl-glucosamine (NAG) and N-acetylmuramic acid (NAM) are linked by β-1, 4-glycosidic bond. These are related to glucose attached with amino acid groups. The structural formulae of NAG and NAM are shown in Fig. 4.7.

A tetra-peptide side chain containing four amino acids (L-alanine, D-glutamate, L-lysine and D-alanine) is attached to each NAM. The third amino acid varies with different bacteria and may be lysine, di-amino-pametic acid or threonine. For example in E. coli instead of L-lysine (the third amino acid) there is mesodiaminopimetic acid.

The D and L forms of amino acids alternate to each other. Except peptidoglycan, the amino acids found in protein are L-forms. The pcarallel tetra peptide side chains are linked by a pentaglycine peptide cross bridge (PPCB) that contains five amino acids (Fig. 4.8).

The PPCB links L-lysine of one tetra-peptide with D-alanine at the terminal end. Due to extensive cross linking the peptidoglycan becomes a rigid macromolecule of the cell wall.

1. Gram-Positive Bacteria:

In most of Gram-positive bacteria, the cell wall contains several layers of peptidoglycan which is inter-connected by side chains and cross bridges (Fig. 4.9B). Peptidoglycan accounts for 40-90% of total dry weight of cell wall. However, the thickness may vary with types of species from 30 nm to 8 nm. The thickness of peptidoglycan provides rigidity to cell wall.

The layers of peptidoglycan are thicker in Gram-positive bacteria than that in Gram-negative bacteria. In most of the Gram-positive bacteria peptidoglycan is associated with acidic polymers containing phosphorus called teichoic acid or acidic polysaccharides such as teichuronic acids. Teichoic acids are hydrophilic, flexible and linear molecules. The presence of teichoic acid makes easy to diagnose the bacteria serologically.

Teichoic acids consist of an alcohol (e.g. glycerol or ribitol) and phosphate. Therefore, it is the polymer of glycerol phosphate or ribitol phosphate.

Teichoic acids are mainly of three types:

(i) Ribitol teichoic acids (found in S.aureus and B.subtilis),

(ii) Glycerol teichoic acid (e.g. B. subtihs), and

(iii) Glucosyl-glycerol phosphate teichoic acid (S. licheniformis). Out of these three only one type is found in a particular bacterium.

The acids are linked to layers of peptidoglycan of plasma membrane. The phosphate groups provide negative charge which in turn controls the movement of cations i e. positive ions across the cells. Teichoic acids possibly play a role in growth of Bacterial cell by regulating the activity of an enzyme autolysin. The acids prevent the extensive break down and possibly the lysis of cell wall. They also store phosphorus.

Peptidoglycan of Staphylococcus aureus consists of linear carbohydrate backbone, the glycan chain. The glycan chain contains alternate residues of NAG and NAM linked by β-1, 4-linkage. The amino acid residues are connected by tetra-peptide and Pentaglycine Bridge (Fig. 4.8).

The wall of most eubacteria contains very low amount of lipid except Mycobacterium and Corynebacterium Mycobacterium exhibits acid fast staining i.e. the stain from cell wall is not easily de-colourized with dilute acid. This is due to the presence of mycolic acid in cell wall. A mycohc acid derivative i.e. trehalose dimycolate plays a role in diseases caused by M.tuberculosis and C. diphthenae.

2. Archaeobacteria:

All archaeobacteria lack murein. Most of them possess cell walls that lack peptidoglycan. Structure and chemical composition of cell wall of archaeobacteria differ from that of eubacteria.

Moreover, the archaeobacteria possess no common cell wall polymer. Usually cell wall is composed of proteins, glycoproteins or polysaccharides. Due to unusual chemical composition the cell envelopes, show a high degree of resistance against cell wall antibiotics and lytic agents.

The cell walls of the Gram-positive archaeobacteria consist of pseudomurein, methanochondroitin, or heteropolysaccharide. All Gram-negative archaeobacteria have cell envelopes which are composed of single layered or more complex crystalline protein or glycoprotein subunits.

In most of the micro-organisms of methanogenic branch and extreme thermophilic sulphur metabolizers, single S-layer is found in cell envelope which provides remarkable resistance. The organ­isms tolerate the extremes of environmental conditions such as high salt, low pH and high temperature.

The glycan chains of S-layer glycoprotein are sulfated, and are similar to eukaryotic proteoglycans. The S-layer shows a complex spongy three dimensional structure, and in some cases it occupies about 30% of the cell envelope.

3. Gram-Negative Bacteria:

The Gram-negative bacteria contain peptidoglycan but in very low amount (Fig. 4.9A). They totally lack teichoic acid. Peptidoglycan is situated in periplasmic space and covalently linked to lipoproteins in the outer membrane.

The periplasmic space is a space between the outer membrane and plasma membrane which appears like gel and contains a high amount of enzymes and transport proteins. Due to the presence of low amount of peptidoglycan, the cell wall of Gram-negative bacteria can easily be disintegrated. The cell envelope of Gram-negative bacteria is a bilayered structure consisting of mainly lipoproteins, lipopolysaccharides (LPS) and phospholipids.

The chemical constituents and arrange­ment are described in detail as below:

(a) Lipoproteins:

Lipoproteins occur freely and in bound forms as well. In lipoproteins of outer membrane, protein binds to lipid non-covalently, whereas in lipoproteins of plasma membrane, proteins bind to lipid covalently.

Lipoproteins have a molecular weight of about 7000 Daltons and consist of about 58 amino acids. Lipoproteins together with matrix proteins form a complex which contains diffusion channels. A diffusion channel is enclosed by three molecules of matrix protein leaving a diameter of about 1.5-2 nm.

(b) Lipopolysaccharides (LPS):

The outer membrane of Gram-negative bacteria is covered by LPS which is made up of polysaccharides covalently linked to lipid A (Fig. 4.10).

Lipid A:

It consists of glucosamine, phosphate and fatty acids. The β-1, 6-D- glucosamine disaccharide units constitute the carbohydrate components of lipid A. The hydroxyl and amino groups of this disaccharide are substituted by the constituents such as polysaccharide chain, phosphate or pyrophosphate and fatty acids. The fatty acids provide hydrophilic property to lipid A.

There are six fatty acids such as lauric acid (12C), myristic acid (14C) and palmitic acid (16C) (present in the ratio of 1:1:1), three molecules of 3-D-hydromynstic acid (14C) two of which contain amide (-NH₂) linkages with each amino group of two glucosamine. The third is esterified through its hydroxyl (-OH) group to myristic acid (Fig. 4.10).

The esterified group of lipid A is referred to as endotoxin which is toxic when present in blood. Lipid A of LPS is responsible to induce fever and shocks.

(c) Polysaccharide:

The polysacharide portion of LPS of Salmonella cell wall is composed of three important components, the inner core, the outer core and the O-antigen side chain. Although the polysaccharide of LPS is also known as O-polysaccharide.

The O-antigen side chains in rough strains of Gram-negative bacteria are absent, whereas the smooth strains of Salmonella have O- antigen side chains which may extend out the wall surface and attain about 30 nm lengths. These chains have antigenic property and, therefore, can be distinguished serologically (e.g. species of Salmonella). Its role is comparable to that of teichoic acids in Gram-positive bacterial cell walls.

The outer membrane consists of two molecules of glucose sand witching a molecule of galactose. One glucose subunit is linked to glucose acetyl and the others to galactose.

The inner core consists of two regions, the keto de-oxyoctonate (KDO) region and di-heptose region. The KDO region comprises of three units of KDO and eight carbon a-keto sugar. The di-heptose region consists of two units of L-glycero-D-manno-heptose, the seven carbon heptose sugar (Fig. 4.10).

(d) Matrix Proteins:

The outer membrane of cell envelope does not warrant the entry of all substances since the nutrients are to pass across the membrane. It is impermeable only to macromolecules such as proteins, lipids, etc. The permeability of outer membrane is due to the presence of proteins called porins that form channels (Fig. 4.9).

The porins are not specific and allow the small molecules. Certain porins are specific and permit only the specific substances such as vitamin B₁₂, nucleotides, etc. Porins also act as receptor sites for bacteriophages and bacteriocins (the proteins produced by certain bacteria that inhibit or kill the related species).

Function of Cell Wall:

Following are the functions of the cell wall (the outer membrane):

(a) Peptidoglycan provides structural integrity to cell by forming a rigid layer in outer membrane. The matrix proteins to some extent also contribute to structure with peptidoglycan.

(b) The cell envelope acts as barrier for diffusion to certain molecules across the envelope,

(c) The matrix proteins act as receptor sites for bacteriophages and bacteriocins.

(d) The O-antigen side chain of polysaccharide of LPS determines the antigen specificity of Gram-negative bacteria.