The Bacterial Cell Envelope

THE CELL ENVELOPE: THE OUTER WRAPPING OF BACTERIA:

Definition:

In bacteria the complex of layers external to the cell protoplasm is called as cell envelope. It includes capsule, slime and cell wall.

(i) Capsule:

Some bacteria produce capsule. It is tightly bound to the cell. Capsule is made up of of repeating polysaccharide units, or of protein, or of both.

(ii) Slime:

It is a loose, soluble cover of macromolecules called as slime capsule. It is present in some bacteria.

Slime provides greater pathogenicity to bacteria and protects them against phagocytosis.

(iii) Cell Wall:

Beneath the extracellular substances and (external to cytoplasmic membrane is cell wall).

It is rigid structure and determines the shape of bacterium.

Cell wall also protect the cells from osmotic lysis.

Cell wall is only absent in mycoplasmas.

Gram Stain Technique on the Basis of Differences in Cell Wall.

Christian Gram developed the technique of gram stain on the basis of differences in the cell wall.

On the basis of this technique bacteria can be divided into two groups.

(i) Gram Positive Bacteria:

These are the group of bacteria  that stained purple. They retain the primary dye due to the formation of CV-I complex in the cell wall.

(ii) Gram Negative Bacteria:

These bacteria are stained pink because they retain secondary dye in the cell wall.

Comparison of Gram Positive and Gram-Negative Cell Wall:

Difference in staining is due to difference in structure of cell walls of two groups. It is clear from the following table:

Characteristics	Gram Positive	Gram Negative
(i) Chemical make up. (ii) Overall thickness (iii) Outer membrane (iv) Periplasmic space (v) Permeability	(a) Peptidoglycan (50% of dry weight in some bacterial cells) (b) Techoic acid (c) Lipotechoic acid (d) Lipids (1-4%) 20-80nm No Present in some More permeable	(a) Lipopolysaccharides (b) Lipoproteins (c) Peptidoglycan 10% dry weight of some bacterial cells. (d) Lipids (11-12%) 8-11nm Yes Present in all Less permeable

Chemical Composition of Cell Wall:

(a) The cell walls of most bacteria have a macromolecules called peptidoglycan.

(b) Its amount varies in different types of bacteria.

(c) Peptidoglycan is composed of long glycan chains cross-linked with peptide fragments.

(d) The cell wall also contains sugar molecules, techoic acid, lipoproteins and lipopolysaccharides. These are linked to peptidoglycan.

Cell Walls of Archaeobacteria:

Cell walls of Archaeobacteria are different from Eubacteria. They do not contain peptidoglycan. Their cell walls are composed of proteins, glycoproteins and polysaccharides.

Cell Walls of Other Bacterial Groups:

Many bacterial group have no cell wall structure which is characteristics of gram positive or gram negative bacteria.

No Cell Wall in Some Bacteria:

Some bacteria have no cell wall (like Mycoplasmas).