First, I must apologise for taking a long time for this video. This video provides detailed information about the structure of Gram positive and Gram negative bacterial cell wall. Please like, comment and subscribe to the channel. Let me understand if any mistakes found in this video through your comments.

Please follow the links below for my previous video on “Structure of bacteria” series.

Part-1: [ Ссылка ]

Part-2: [ Ссылка ]

Vector Graphics: Adobe Illustrator

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Welcome to the part three of “structure of bacteria” series. In the previous video we discussed about the structures external to the cell wall. In this video let’s understand the bacterial cell wall.

Cell wall is an external layer present just above the cell membrane. Cell wall is present in all bacteria except Mycoplasma. It is a rigid structure that gives a shape to the bacterial cell. Based on the cell wall structure, the bacteria are divided in to two groups, called gram positive and gram negative bacteria.

The composition of gram positive bacterial cell wall is completely different from gram negative bacteria. Lets understand the differences, and the arrangement of bio molecules.

Peptidoglycan layer:

Peptidoglycan layer also called as Murein, is common component of cell wall of gram positive and negative bacteria. It is composed on two major parts. Glycan portion and Peptide portion.

The Glycan portion of peptidoglycan forms the backbone. The glycan portion is composed of N Acetyl glucosamine and N acetyl muramic acid. These two sugars are bound by β bond.

Tetra peptide side chain

The Carboxyl group of N acetyl muramic acid is connected to a tetra peptide side chain. This tetra peptide side chain is composed of four amino acids, which include

L-alanine, D-Glutamic acid, L-Lysine or Di amino pimelic acid and D-alanine.

Peptide Bridge

The tetra peptide side chains of two adjacent Mureins are linked by a peptide bridge.

In gram negative bacteria, this cross link is formed directly between the third amino acid, which is Di amino pimelic acid and D-Alanine, which is the fourth amino acid.

In gram positive bacteria, this cross link is formed between D-Alanine and L-Lysine by five Glycine amino acids.

The Gram positive cell wall

The cell wall of Gram positive bacteria is thick. The major part that is 90 percent of gram positive bacterial cell wall is composed of peptidoglycan layer. The remaining 10 percent consists of proteins, polysaccharides and teichoic acid.

Teichoic acid is either connected to N acetyl muramic acid of peptide glycan or to the Lipids in the cytoplasmic membrane. Teichoic acid bound to the lipids, is called Lipo teichoic acid or LTA. Teichoic acids covalently bound to the peptide glycan of cell wall are called wall teichoic acid or WTA.

Uses

Teichoic acids provide rigidity to the cell wall and also regulates the cell wall rupture by auto lysine enzymes. Teichoic acids are found only in gram positive bacteria and absent in gram negative bacteria.

The Gram negative cell wall

The structure of The Gram negative cell wall is more complex than gram positive bacterial cell wall.

External to the thin and single layered peptide glycan, an outer membrane is present. The space between the outer membrane and the inner plasma membrane is called Periplasmic space.

This periplasmic space is also present in gram positive bacteria. However, it is very smaller when compared with gram negative bacteria.

The substance present in the periplasmic space is called the Periplasm. Many proteins and enzymes are present in the periplasm. Periplasm hosts bio chemical reactions that cause degradation, transport and motility.

The outer membrane can be found only in gram negative bacteria and absent in gram positive bacteria. The outer membrane contains proteins, phospholipids, and lipo polysaccharides.

Lipo polysaccharides are unique components of gram negative bacteria and are referred as Endotoxins.

The outer membrane also contains large quantities of Braun’s lipo protein. It is covalently bound with the peptidoglycan layer present underneath, by its hydro phobic end. Braun’s lipo protein strongly holds the peptidoglycan layer and outer membrane.

The other proteins present in the outer membrane are Porins. Porins form a narrow channel and allow different molecules pass through.

The lipo polysaccharides are complex molecules composed of three regions. Lipid-A. Core polysaccharide. And O-Antigen.

As i said earlier, there are lot of differences in the cell wall of gram positive and gram negative bacteria. Let’s have a look at them.

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