Sunday 20 May 2012

Structure Of Bacteria

Introduction

Bacteria when observed under the electron microscope, shows detail cell structure.
The bacteria has a nucleus without a nuclear membrane.
Based on the staining technique of Christian Gram the bacteria are classified into two types.
  1. Gram positive.
  2. Gram negative.
Bacteria shows the following structures.

Cell Wall
  • The bacterial cell is enclosed by a definite and complex cell wall.
  • The cell composes of several peptidoglycan or mucopeptide layers.
  • The cell wall of gram positive bacteria contains teichoic acid. In gram negative bacteria it is absent.
Functions Of Cell Wall
  1. It gives definite shape and size to the cell.
  2. It protects the cell from toxic materials.
  3. It protects the cell from osmotic lysis.
  4. In some bacteria the components of the cell wall lead to its pathogenic nature.

Gram Positive

A gram positive bacteria is almost similar to gram negative bacteria internally. but, it is totally different in its structure externally. It has cytoplasmic lipid membrane. It has a thick peptidoglycan layer which is made up of mucopeptides, glycopeptides, mureins and which constitutes about 95% of the total cell wall. Teichoic acids and lipoids are present which link to form  lipoteichoic acids. These lipoteichoic acids act as chelating agents and help in attachment to certain substances. The function of teichoic acids is not completely known yet.


Gram Negative

It has a cytoplasmic membrane. It has a thin peptidoglycan layer compared to gram positive bacteria. Outer membrane consists of Lipopolysaccharides. There is a space between the layers of peptidoglycan and the secondary cell membrane called the periplasmic space.

Structures Outside the Cell Wall
The structures present outside the cell wall help in protection, attachment to other objects and movement of the cell.

Glycocalyx
  • It is a sugar coat formed by the network of polysaccharides.
  • It is present just outside the cell wall.
  • It is also known as the bacterial capsule.
  • It protects the bacteria from phagocytes.

Flagella

  • It is 20nm in its thickness.
  • It is made up of a protein called flagellin.
  • It is hollow, cylindrical and helical in shape.
  • The flagellin is responsible for the helical structure of the flagella.
  • It helps in locomotion.

Pilli

  • It is a thin, hollow, cylindrical, hairlike structure.
  • It is similar to flagella.
  • It does not promote locomotion.
  • It is helps in attachment to surfaces.
  • Some pilli called as sex pilli help in the transfer of genetic material.