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The Bacterial Cell Wall
The cell wall is the outer most rigid structure which provides the structure to the bacterial cell. Moreover, it helps the cell by providing protection to the cell from osmotic lysis, toxic chemicals, and other pathogens. The chemical structure of the bacterial cell wall is similar in different bacteria. However, the chemical organization of the cell wall differs in the case of gram-positive and gram-negative cell wall.
Peptidoglycan which is otherwise known as murin is a polymer-like structure which is made up of two types of sugars such as N-acetylglucosamine and N-acetylmuramic acid. Different amino acids are connected with these sugar subunits. There are three unusual amino acids present in the cell wall structure such as D glutamic acid, D alanine, and mesodiaminopimelic acid. The backbone of the cell wall is made up of alternative N acetylglucosamine and N-acetylmuramic acid subunits. A peptide chain of D and L amino acids are attached to the carboxyl group of N acetylmuramic acid. The rigid structure is formed by the cross-linking of the peptides. The carboxyl group of the terminal D alanine is connected to the amino group of diaminopimelic acid. However, in some cases, a peptide bridge is formed between two sugar subunits.
Gram-positive cell wall
- The main structure of the gram-positive cell wall contains a thick peptidoglycan layer. The peptidoglycan layer contains peptide interbridge. Apart from the peptidoglycan, a major constituent of gram-positive cell wall is Teichoic acid. Teichoic acid is made up of long-chain of glycerol or ribitol which is connected by phosphate groups. Amino acids like D alanine and sugars like glucose is often attached to the glycerol units. The teichoic acid is covalently linked with the peptidoglycan layer or the plasma membrane. Teichoic acids are negatively charged and appear at the surface of the peptidoglycan layer. Due to this, the gram-positive cell wall is also negatively charged.
- A small periplasmic space is present in the gram-positive cell wall between the plasma membrane and cell wall. Periplasm is present at the periplasmic space which contains few proteins and exoenzymes.
- A protein layer is often found on the peptidoglycan layer of some bacteria like Staphylococcus sp. Some of the proteins present in this layer interact with the surrounding environment and some of the proteins are linked with peptidoglycan or teichoic acid. S layer proteins are an example of this kind of protein. Apart from these proteins like M protein functions as virulence factors are also present in this layer.
Gram-negative cell wall
- The peptidoglycan layer in gram negative cell wall distinctively differs from the gram positive cell wall. The peptidoglycan layer is relatively thin in case of gram negative cell wall. Apart from that the petidoglycan layer, the periplasmic space in gram negative cell wall also differs from the gram positive cell wall.
- The periplasmic space in gram negative cell wall contains around 20-40% of the total cell volume. There are different proteins present at the periplasmic space such as hydrolytic enzymes, transport proteins, etc. Apart from that, some proteins present in the periplasmic space of some denitrifying bacteria which involves the conversion of nitrate to nitrogen gas and some are also involved in peptidoglycan synthesis and modification of toxic compounds.
- An outer membrane is present outside of the peptidoglycan. This layer is attached to the cell in two different ways such as by Braun’s lipoprotein and by adhesion sites. The Broun’s lipoproteins are connected to the peptidoglycan layer and embedded in the outer layer by its hydrophobic end. In the case of the adhesion sites, the two different membranes are directly connected.
- Lipopolysaccharides are an essential part of the lipoproteins. These subunits are made up of both lipid and carbohydrate residues. The structure of the lipoprotein is divided into three parts, Lipid A, core polysaccharide and O side chain. Lipid A is made up of two glucosamine sugar units each of which are connected with fatty acids and phosphate. The core polysaccharide group is connected with the lipid A region. In this place several types of sugars are present. O side chain or the O antigen is a polysaccharide chain which is present at the outer surface of the layer.
- Lipopolysaccharide functions as the attachment site of different bacteria. Moreover, it also works as a permeability barrier which restricts the entry of toxic molecules. The O antigen involves in producing an immune response. The lipid A portion is chemically toxic and worked as endotoxins.
- Despite the permeability barrier, there are some molecules that can transport inside the cell due to the presence of porin proteins. These are a cluster of trimers present at the outer surface. Molecules ranging from 600 to 700 Daltons can pass through these proteins.