Sunday, 26 July 2020

Cell Wall Composition of Gram Positive and Gram Negative Bacteria


  Cell wall forms a fairly rigid layer just outside the plasma membrane in most of the prokaryotes . The cell wall accounts for 10 to 40 % of the cellular dry mass depending on the growth conditions . It provides the cell support , and protection against mechanical stress or damage from osmotic rupture and lysis . The major component of the bacterial cell wall is peptidoglycan or murein . This rigid structure of peplidoglycan, specific only to prokaryotes. gives the cell shape and surrounds the cytoplasmic membrane.

Overall structure of Cell wall

1].  Light microscopic examination of bacteria, does not permit observation of differences between the cell walls of gram - positive and gram negative bacteria. It is possible only when the cell walls are examined under the transmission electron microscope.

2].  The wall of gram - positive bacteria consists of a single homogeneous layer of peptidoglycan for murein ) . that remain outside the plasma membrane. It is 20 to 80 nm thick .

3]. The cell wall of gram - negative bacteria consists of  one layer of outer membrane ( 7 to 8 nm thick ) and one layer of peptidoglycan ( 2 to 7 nm thick )

4].  A space is observed between plasma membrane and outer membrane of cell wall. It is called periplasm. Periplasm of gram - negative bacteria contains hydrolyzing enzymes. They facilitate the movement of molecules across the membrane.

5].  The gram - positive bacterial cell wall contains peptidoglycan and telchole acid as the major constituents, whereas that of gram - negative bacteria possesses peptidoglycan. lipopolysaccharide, lipoproteins and phospholipids.


Peptidoglycan

1].  Peptidoglycan ( also called murein ) is the most Important constituent of prokaryotic cell wall. It is relatively porous, elastic and somewhat stretchable. It is a complex mesh - like polymer of repeating subunits. Variation occurs from species to species in its chemical composition and structure. However, the basic structure is same in all

2]. Peptidoglycan. a hetero- polysaccharide, consists of repeating disaccharide units, to which a short peptide chain attaches and forms a complex mesh ( lattice ). The disaccharide part is formed by two acetylated amino sugars, N - acetyl glucose amine ( NAC ) and N - acetyl muramic acid ( NAM, the lactyl ether of N - acetyl glucose amine ), that are joined alternately by b-1, 4 glycosidic linkage.

3]. NAG and NAM Join linearly to form a row of 10 - 65 sugars and forms the carbohydrate backbone. A short peptide tetrapeptide - a chain of four amino acids. consisting of L - alanine, D - glutamic acid , Di amino plmelic acid ( DAP ) or L - lysine and D - alanine , attaches to the NAM residue of backbone. Tetra peptide contains alternating pattern of D- and L - amino acids.

4]. In gram-positive bacteria, these tetra peptides are Joined by cross-links in order to form a strong. mesh like polymer. Normally the carboxyl (-COOH) group of terminal D-alanine joins directly with DAP or L lysine of other tetra peptide by a penta peptide inter-bridge. Most gram-negative bacteria lack a peptide inter-bridge. Peptide inter-bridge does not always exist between all tetra peptides.

Components of gram-positive cell walls

1]. The cell walls of gram-positive bacteria are thick and made up of several layers of peptidoglycan (as many as 40). In addition, they possess a large amount of another macromolecule called teichoic acid.

2]. Teichoic acids are polymers of glycerol or ribitol joined by phosphate groups. Amino acids like D-alanine or sugars like glucose are attached to the glycerol or ribltol groups. The teichoic acids may be covalently linked with either peptidoglycan in the wall or lipids in the plasma membrane. They are respectively called wall teichoic acid or Lipoteichoic acid.

3]. Teichoic acids are negatively charged. so they contribute to the negative charge of the cell surface. They may be involved in :
a). Regulation of entry or exit of
        molecules
b). Prevention of cell lysis
c). Antigenicity
d). Attachment of
       bacteriophages

Components of gram-negative bacterial cell walls

The cell walls of gram-negative  bacteria are relatively more complex. It possesses a thin layer of peptidoglycal, surrounded by outer membrane, made of lipopolysaccharide.

Outer Membrane

The outer membrane consists of lipopolysaccharides (LPS). It consists of Lipid A, Core polysaccharide and O side chain. The outer membrane is linked to inner peptidoglycan layer by a unique lipoprotein, called Braun's lipoprotein.

a). Lipid A  is not a glycerol lipid. It consists of two glucosamine sugar derivatives, each attached with three fatty acids and phosphates or pyrophosphates. The long chain fatty acids, which may be caproic acid, lauric acid, myristic acid, palmitic acid or stearic acids, join to glucosamine (GKN) phosphates through esteramine linkage. The disaccharides attaches to the core polysaccharide and the fatty acids attach to the outer membrane.

b). Core polysaccharide consists of an oligosaccharide unit attached to the glucosamine residues of lipid-A. It consists of keto-deoxyoctonate (KDO), seven-carbon sugars (heptose). glucose, galactose and N-acetyl glucosamine.

c). O-polysaccharide, also called O-side chain or O-antigen, extends from the core. It contains several peculiar sugars e.g. galactose (Gal), glucose (Gui), rhamnose (Rha) and mannose (Man) They are connected in four or five-member sequences, which are usually branched. Repetition of sugar sequences forms a long O-polysaccharide.

LPS contains porin proteins. It is tube shaped. It permits the passage of molecules smaller than 600 to 700 Daltons.

Inner Membrane

1]. Inner membrane of gram - negative bacteria consists of a thin layer of peptidoglycan . The peptidoglycan layer is non - covalently anchored to lipoprotein molecules called Braun's lipoproteins through their hydrophobic head .

2]. Sandwiched between the outer membrane and the plasma membrane , a concentrated gel - like matrix ( the periplasm ) is found in the periplasmic space . This periplasmic space contains binding proteins for transport of nutrients in to the cell . The periplasm space can act as reservoir for virulence factors and a dynamic flux of macromolecules representing the cell's metabolic status and its response to environmental factors .

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