Introductory Microbiology

Cell wall

 
• In the gram +ve bacteria, the cell envelope contains mainly the peptidoglycan and the techoic acids. The peptidoglycan is a substituted sugar polymer found only in prokaryotic organisms. It consists of two acetylated amino-sugars, n-acetyl glucosamine and n-acetyl muramic acid and a small number of amino acids mainly D-glutamic and D and L-alanine. The two amino-sugars forms ß,1-4 linkage and each strand contains 10 to 65 disaccharide residues. Short peptides containing 4 amino acids are attached to the muramic acid residues and the most common sequence is L-alanine, D-glutamic acid, L-diaminopimelic acid D-alanine.
• The third amino acid varies with different bacteria and may either be lysine, Di-aminopimelic acid or threonine. Cross linked by varying number of amino acids.

Peptidoglycan layer of Staphylococcus aureus

• In the cell envelopes of gram -ve bacteria, two distinct layers are recognized. One, a uniform inner layer about 2-3 nm wide and a thicker outer layer about 8-10 nm wide. The outer layer also called the outer membrane and the peptidoglycan is confined to the inner layer. The outer membrane contains proteins, lipoproteins and lipopolysaccharides.

Structure and chemical composition of cell walls of Gram-negative (A) and Gram-positive (B) bacteria

Fluid mosaic model of bacterial membrane

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