Cell Wall

Bacterial cells are surrounded by a complex cell wall that protects against osmotic lysis. Cell wall is basically made up of peptidoglycan.

Cell wall of Gram positive bacteria

Gram positive bacterial cell wall is mainly composed of peptidoglycan; 20 to 80 nm thick homogenous layer of peptidoglycan. Peptidoglycan often contains peptide interbridge of short amino acid chain. Cell wall also contains large amount of an acidic substance called teichoic acid, a polymer of ribitol or glycerol joined by phosphate group. Teichoic acids are negatively charged and responsible for providing negative charge to the cell wall. Teichoic acid are either linked directly to N-acetylmuramic acid (NAM) of peptidoglycan or to the lipids of the plasma membrane. Teichoic acids linked to the lipids are known as lipoteichoic acid. Teichoic acid provides rigidity to the cell wall thereby maintaining the cell wall structure. Although the lipid content of Gram positive bacteria is very less but some like Mycobacteria and Cornyebacterium sp are rich in lipids known as Mycolic acid. The outer membrane and lipopolysaccharide is absent in Gram positive bacteria.

Cell wall of Gram negative bacteria

As compare to Gram positive, Gram negative bacteria has much more complex cell wall. They have a thin peptidoglycan layer about 2-7 nm (in E.coli, it is 2 nm) and contain a 7-8 nm thick outer membrane. The space between the two i.e. between plasma membrane and outer membrane is called periplasmic space and it contains periplasm. The outer membrane and peptidoglycan are linked by Braun’s lipoprotein.

The other characteristics component of Gram negative bacterial cell wall is lipopolysaccharides. They are the complex of lipids and polysaccharides and are present in outer membrane. They provide chemical protection against the action of lipophilic antibiotics and bile salts and also contribute to the structural integrity of bacteria.

The outer membrane acts as a protective barrier and allows the entry of only few molecules. But the outer membrane is more permeable as compare to the plasma membrane due to the presence of special porin proteins that permits small molecules like glucose and other monosaccharides.

LPS (Lipopolysaccharide)

LPS are large complex molecules containing both carbohydrate and lipid and are generally found in the outer membrane of Gram negative bacteria. It comprises of three parts: 1) lipid A, (2) the core polysaccharide, and (3) the O side chain.

Lipid A: It consists of two phosphorylated glucosamine sugar derivatives, each linked with three fatty acids. LPS projects from the cell surface except the hydrophobic fatty acid chain which is buried in the outer membrane. The toxicity of the Gram negative bacteria is mainly due to lipid A, making LPS to act as an endotoxin. When fragments of lipid A containing membrane are released into circulation due to the cell lysis by immune system they causes fever, diarrhea and other symptoms.

Core polysaccharide: It is linked to lipid A. It commonly contains sugar like heptose and keto-deoxyoctulosonate (KDO or 3-Deoxy-D-manno-oct-2-ulosonic acid). Non-carbohydrate components like amino acids and phosphate are found in the core of many bacteria. The presence of charged sugar and phosphate in the core makes LPS to contribute to the negative charge on the bacterial surface.

O side chain: It is also termed as O-polysaccharide or O-antigen. It is a polysaccharide chain, attached to the core polysaccharide and extends outward. It also contains some unusual sugar molecules. Its composition varies between different strains of Gram negative bacteria. Presence of the full length O-antigen makes LPS smooth whereas absence or incomplete O-antigen makes LPS rough. They functions as an antigen thereby acting as a target for host antibodies. 

Note: Mycoplasma lacks cell wall.