Microbiology – Cell Wall of Gram Positive Bacteria.

Introduction

  • A bacterial cell shows various parts and these parts have specific structure and functions.
  • These parts are separated by a rigid cell wall some parts are on internal side of cell wall and some parts are on external side of cell wall.
  • The parts present on internal side of cell wall are cell membrane, cytoplasm, nuclear region, reserve food material etc.
  • The cellular parts present on external to the cell wall are flagella, capsule and pilia.

Structure and function of Cell wall

  • Cell wall is outer covering of bacterial cell in absence of capsule.
  • These cell wall is rigid in nature which imparts a proper shape to bacteria.
  • Cell wall encloses all internal parts of cell.
  • On the basis of typical structure,composition and Gram staining they are classified

A)   Gram positive bacteria.

B)   Gram negative bacteria. Types of bacterial cell

Types of bacterial cell

A) Cell wall of Gram positive bacteria.

  •  It is thick monolayered and accounts about 10 to 50% dry weight of cell.
  • Gram positive cell wall is mainly composed of two important components.

     1. Peptidoglycan.    

     2. Teichoic acid.

1. Peptidoglycan  

Components of peptidoglycan of Gram positive cell.

Components of Peptidoglycan of Gram positive cell wall
Components of Peptidoglycan of Gram positive cell wall.
  • Peptidoglycan is important component of cell wall.
  • It is also called as glycopeptide and mucopeptide.
  • Peptidoglycan is present almost in all bacteria except Halococcus and Halobacter.
  • The three main components of peptidoglycan are

a ) Glycan backbone strand.

b ) Tetrapeptide side chain.

c ) Cross bridges.

Structure of peptidoglycan of gram positive cell wall.

Peptidoglycan structure of Gram positive cell wall
Peptidoglycan structure of Gram positive cell wall

a ) Glycan backbone strand.

  • Glycan backbone strand contains a long chain of amino sugars these sugars are              N-acetylmuramic acid and N-acetyl glucosamine.
  • These sugars are linked to each other by Beta-1,4 linkage.
  • It is the unbranched chain.

b ) Tetrapeptide side chain

  • Tetrapeptide side chain consist of four amino-acids in a particular sequence.
  • The seguence is L-alanine, D-glutamic acid, L-lysin, D-alanine.
  • Each N-acetylmuramic acid of glycan strand is linked to four amino acid by peptide bond.
  • It is called as tetrapeptide side chain.

c ) Cross linkage

  • When two adjacent tetrapeptide side chain are linked to each other by a glycan molecule called as cross linkage.
  • The carboxyl group of fourth amino aid D-alanine of side chain is linked to amino group third amino acid L-lysin of second tetrapeptide side chain.
  • The linkage of carboxyl group and amino group is formed by five glycan molecules so called as cross bridge.

2. Teichoic acid

  • Teichoic acid is he second most important component of Gram positive bacterial cell wall.
  • Teichoic acid is a water soluble polymere .
  • It has repeating sugars.
  • Depending on types of repeating sugars teichoic acid is of two types.
  • They are Glycerol teichoic acid and Ribitol teichoic acid.

    Components of teichoic acid
    Components of teichoic acid of Gram positive cell wall

1.Glycerol teichoic acid 

  • It is a long chain of glycerol 3 phosphate molecule.
  • These  glycerol 3 phosphate are linked by phosphodiester bonds.

2. Ribitol teichoic acid.

  • It is a long chain of ribitol 5 phosphate molecule.
  • These are also linked by  phosphodiester bonds.

Functions of cell wall.

  • Cell wall gives rigidity, specific size and shape to cell.
  • Cell wall protects cell from osmotic pressure.
  • In high salt concentration cell wall protects cell from explosion.
  • It plays important role in growth and cell division.
  • Cell wall components such as lipopolysaccharides and teichoic acid provides attachment site for bacteriophages.
  • Cell wall is a permiable membrane and because of its permiability it provides transport of salts,water and other nutrients in the cell.

 

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