The cell wall is a tough and rigid structure, surrounding the bacterium. It is 10–25 nm in thickness and weighs about 20–25% of the dry weight of the cell. The cell wall has following functions:
- It provides protection to the cell against osmotic lysis
- It confers rigidity upon bacteria due to presence of peptidoglycan layer in the cell wall
- The cell wall can protect a cell from toxic substances and is the site of action of several antibiotics
- Virulence factors: Bacterial cell wall contains certain virulence factors (e.g. endotoxin), which contribute to their pathogenicity
- Immunity: Antibody raised against specific cell wall antigens (e.g. antibody to LPS) may provide immunity against some bacterial infection.
Gram-positive Cell wall
Cell wall of gram-positive bacteria is simpler than that of gram-negative bacteria (Table 1).
Table1. Differences between gram-positive and gram negative cell wall.
Peptidoglycan
In gram-positive bacteria, the peptidoglycan layer is much thicker (50–100 layers thick, 16–80 nm) than gram-negative cell wall (Fig. 1).
- Each layer is a mucopeptide (murein) chain, composed of alternate units of N-acetyl muramic acid (NAM) and N-acetyl glucosamine (NAG) molecules; cross linked to each other via tetrapeptide side chains and pentaglycine bridges
- A tetrapeptide side chain ascended from NAM molecule is composed of L-alanine—D-glutamine—L-lysine—D alanine
- The L-lysine of one tetrapeptide chain is covalently linked to the terminal D-alanine of the adjacent chain via a pentaglycine bridge (Fig. 2).
Fig1. Structure of gram-positive cell wall.
Fig2. Peptidoglycan layer of gram-positive cell wall.
Teichoic Acid
Gram-positive cell wall contains significant amount of teichoic acid which is absent in gram-negative bacteria. They are polymers of glycerol or ribitol joined by phosphate groups. These molecules may maintain the structure of cell wall. They are of two types: (i) Cell wall teichoic acid and (ii) Lipoteichoic acid.
Gram-negative Cell wall
Gram-negative cell wall is thinner and more complex than the gram-positive cell wall, comprises of the following components (Fig. 3 and Table 1).
Fig3. Gram-negative cell wall.
Peptidoglycan Layer
It is very thin (1–2 layer, 2 nm thick), composed of a mucopeptide chain similar to that of gram-positive cell wall, and consists of alternate NAM and NAG molecules. However, it differs from the latter by (Fig. 4):
- Meso-diaminopimelic acid is present at third position of the tetrapeptide side chain ascended form NAM molecule
- The pentaglycine bridge is absent
- The tetrapeptide side chains are directly linked to each other by the covalent linkage between D-alanine of one chain with meso-diaminopimelic acid of the adjacent chain.
Fig4. Peptidoglycan layer of gram-negative cell wall.
Outer Membrane
This is a phospholipid layer which lies outside the thin peptidoglycan layer; firmly attached to the latter by a membrane protein called Braun’s lipoprotein.
- It serves as a protective barrier to the cell
- Outer membrane proteins (OMP) or porin proteins: They are the specialized proteins present in outer membrane; help in transport of smaller molecules.
Lipopolysaccharide (LPS)
This layer is unique to gram-negative bacteria, which is absent in gram-positives. It consists of three parts:
- Lipid A or the endotoxin: It has endotoxic activities, such as pyrogenicity, lethal effect, tissue necrosis, anticomplementary activity, B cell mitogenicity, adjuvant property and antitumor activity
- Core polysaccharide: It is projected from lipid A region. It is composed of 10–12 sugar moieties
- O side chain (or O antigen or somatic antigen): It is a polysaccharide chain extending outwards from the core polysaccharide region.
* It greatly varies in composition between bacterial strains. Bacterial strains vary in their composition of O antigen; a property which is used for serotyping for bacteria
* O antigen is also a major surface antigen, induces antibody formation.
Periplasmic Space
It is the space between the inner cell membrane and outer membrane. It encompasses the peptidoglycan layer.
