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.

مواضيع ذات صلة

The Growth Curve in Batch Culture

The Measurement of Microbial Concentrations

Bacterial Pathogenesis

Distribution of Normal Flora in The Body

Cultivation of Bacillus and Similar Organisms

Diagnostic Laboratory Tests for Borrelia Burgdorferi

Bacillus cereus

Bacillus anthracis

Antimicrobial Susceptibility Testing and Therapy for Streptococcus,Enterococcus, and Similar Organisms

Serodiagnosis Of Streptococcus,Enterococcus, and Similar Organisms

Approach To Identification of Streptococcus, Enterococcus, and Similar Organisms

Criteria For Identification of Bacteria