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Thursday, September 23, 2010

Cell wall of bacteria is defined as the rigid structure present below the capsules,sheaths and flagella and external to the cell membrane of the cell that gives shape to the cell.bacteria
Cell wall accounts for 10-40% of dry weight of the cell.On the basis of composition, bacteria can be divided into two major groups i.e.gram positive and gram negative bacteria.
The original distinction between this two bacteria is based on the gram staining reaction.The rigid layer in the cell wall of bacteria is called as peptidoglycan or merein.

Each layer of peptidoglycan is a thin sheets of two sugar derivatives,N acetyl glucosamine and N acetyl muramic acid and a small group of tetrapeptide consisting of L-alanine,D-alanine,D-glutamate and and either lysine or diaminopimelic acid.
The rigidity of the cell wall is provided by glycosidic bond between two sugar derivatives.An unusual feature of bacterial cell wall is the presence of two amino acids that have D-configuration,D-alanine and D-glutamic acid since amino acids are always in L-isomeric form.
Cell wall of gram positive bacteria
Peptidoglycan,content is high in gram positive bacteria as 70% of the dry weight of its cell wall,is of thickness 10-20nm.Besides peptidoglycan cell wall contains polysaccharides that are covalently linked to peptidoglycan e.g.Streptococcus pyogens also the wall of Streptococcus aureus contains teichoic acid.
Teichoic acid is an acidic polysaccharide composed of ribitol phosphate or glycerol
Phosphate.Teichoic acids are negatively charged that binds with magnesium ions and thus protect from thermal injury.Teichoic acid binds with lipids which are known as lipoteichoic acids.Lipid content is less in gram positive bacteria except Mycobacterium,corynebacterium etc.These bacteria are rich in mycolic acid.

Gram negative bacteria
Peptidoglycan layer is much thinner.Lipopolysaccharide is present in outer membrane.also in outer membrane porins are present which act as pores for particular molecules.There is a space between the layers of peptidoglycan and the secondary cell membrane called the periplasmic space.Flagella if present have four supporting rings instead of two.Techoic acids or lipotechoic acids are not present as in gram positive bacteria.
Lipoproteins are attached to the polysaccharide bone.Most of the gram negative bacteria do not sporulate.


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