Chemical Structure of Bacterial Cell Wall
Chemical Structure of Bacterial Cell Wall is composed of mucopeptide(peptidoglycan or murein) formed by N-acetyl glucosamine and N-acetyl muramic acid molecules alternating in chains, which are cross-linked by peptide bonds
Peptidoglycan consists of three parts
1. The backbone is composed of alternating N-acetylglucosamine and N-acetylmuramic acid.
2. A set of identical tetrapeptide side chains attached to N-acetylmuramic acid.
3. A set of identical pentapeptide cross-bridges.
Several antibiotics interfere with the construction of the cell wall peptidoglycan. In gram-positive bacteria, the cell wall consists mainly of peptidoglycan and teichoic acids, whereas in gram-negative bacteria, the cell wall is more complex in both the anatomical and chemical sense and includes the thinner peptidoglycan and outer membrane.
Difference Between Cell Wall of Gram-positive and Gram-negative Bacteria
In general, the walls of the gram-positive bacteria have simpler chemical nature than those of gram-negative bacteria.
Gram-positive Bacterial Cell Wall
The gram-positive bacterial cell wall is about80 nm thick and is composed mostly of several layers of
Peptidoglycan: It constitutes 50-90 per cent of the dry weight of the wall and is thicker and stronger (more extensively cross-linked) than those of gram-negative bacteria (peptidoglycan comprising 5-10 per cent of the wall material). By contrast, gram-negative cell walls contain only a thin layer of peptidoglycan.
Teichoic acid: In addition, the cell walls of gram-positive bacteria contain teichoic acids, which consist primarily of alcohol (such as glycerol or ribitol) and phosphate. There are two types of teichoic acids: wall teichoic acid, covalently linked to peptidoglycan, and membrane teichoic acid (lipoteichoic acid), covalently linked to membrane glycolipid and concentrated in mesosomes. Some gram-positive species lack wall teichoic acids, but all appear to contain membrane teichoic acids
Gram-negative Cell Wall
The gram-negative cell wall is structurally quite different from that of gram-positive cells. It consists
of peptidoglycan and, lipoprotein, outer membrane, and lipopolysaccharide.
i. Peptidoglycan layer
The peptidoglycan layer is a single-unit thick and constitutes 5-10 per cent of the dry weight of the wall of gram-negative bacteria. It is bonded to lipoproteins covalently in the outer membrane and plasma membrane and is in the periplasmic, a gel-like fluid between the outer membrane and plasma membrane. The periplasmic contains a high concentration of degradative enzymes and transport proteins. The periplasmic space is approximately 20 to 40 per cent of the cell volume, which is far from insignificant.
Lipoprotein or murein lipoproteins seemingly attach (both covalently and noncovalently) to the peptidoglycan by their protein portion, and to the outer membrane by their lipid component.
Function: To stabilize the outer membrane and anchor it to the peptidoglycan layer.
iii. Outer membrane
External to the peptidoglycan, and attached to it by lipoproteins in the outer membrane. It is a bilayered
structure. Its inner leaflet is composed of phospholipid while phospholipids of the outer leaflet are replaced by lipopolysaccharide (LPS) molecules.
a. A protective barrier: A most important function is to serve as a protective barrier. It prevents or slows the entry of the salts, antibiotics and other toxic substances that might kill or injure the bacterium.
b. Porins of transmembrane proteins: In addition to LPS, the outer membrane also contains several important proteins that function in the selective transport of the nutrients into the cell. Porins or
transmembrane proteins, traverse the outer membrane and form trimeric channels that permit the
passage of molecules such as nucleotides, disaccharides, peptides, amino acids, vitamin B12, and iron.
iv. Lipopolysaccharide (LPS)
A structural component that is unique to the gram-negative outer membrane is lipopolysaccharide (LPS). It is a large complex molecule that contains lipids and carbohydrates and consists of three components:
a. Lipid A is the lipid portion of LPS and is embedded in the top layer of the outer membrane. When gram-negative bacteria die, they release Lipid A, which functions as an endotoxin. All the toxicity of the
endotoxin is due to lipid A which is responsible for the endotoxic activities, that is, pyrogenicity, lethal
the effect, tissue necrosis, anticomplementary activity, B-cell mitogenicity, immunoadjuvant property and anti-tumour activity.
b. The core polysaccharide is attached to lipid A and a terminal series of repeat unit contains unusual sugars. Its role is to provide stability
c.O polysaccharide: Extends outward from the core polysaccharide and is composed of sugar molecules. O polysaccharides function as antigens and are useful for distinguishing species of gram-negative bacteria. The role is comparable to that of teichoic acids in gram-positive cells. Polysaccharide represents a major surface antigen of the bacterial cell. It is known as O antigen. Bacteria carrying LPS containing O antigen form smooth colonies in bacteriological media in contrast to those lacking the O antigen, which forms rough colonies.