Peptidoglycan
Peptidoglycan or murein is a polymer consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane of most bacteria, forming the cell wall. The sugar component consists of alternating residues of β-(1,4) linked "N"-acetylglucosamine (NAG) and "N"-acetylmuramic acid (NAM). Attached to the "N"-acetylmuramic acid is a peptide chain of three to five amino acids. The peptide chain can be cross-linked to the peptide chain of another strand forming the 3D mesh-like layer. Peptidoglycan serves a structural role in the bacterial cell wall, giving structural strength, as well as counteracting the  osmotic pressure of the cytoplasm. Peptidoglycan is also involved in binary fission during bacterial cell reproduction. This repetitive linking results in a dense peptidoglycan layer. This peptidoglycan layer is critical for maintaining cell form and withstanding high osmotic pressures, and it is regularly replaced by peptidoglycan production. Peptidoglycan hydrolysis and synthesis are two processes that must occur in order for cells to grow and multiply. This technique is carried out in three stages: clipping of current material, insertion of new material, and re-crosslinking of existing material to new material. The peptidoglycan layer is substantially thicker in Gram-positive bacteria  (20 to 80 nanometers) than in Gram-negative bacteria (7 to 8 nanometers). Depending on pH growth conditions, the peptidoglycan forms around 40 to 90% of the cell wall's dry weight of Gram-positive bacteria but only around 10% of Gram-negative strains. Thus, presence of high levels of peptidoglycan is the primary determinant of the characterisation of bacteria as Gram-positive. In Gram-positive strains, it is important in attachment roles and serotyping purposes. For both Gram-positive and Gram-negative bacteria, particles of approximately 2 nm can pass through the peptidoglycan. It is difficult to tell whether  an organism is gram-positive or gram-negative using a microscope; Gram staining, created by Hans Christian Gram in 1884, is required. The bacteria are stained with several dyes such as crystal violet, iodine alcohol, and safranin using the gram staining procedure. Gram positive cells are purple after staining, while Gram negative cells are red. The peptidoglycan types of bacterial cell walls and their taxonomic implications have been described by Schleifer and Kandler. Archaea do not contain peptidoglycan (murein). Some Archea contain pseudopeptidoglycan (pseudomurein).    Structure. The peptidoglycan layer in the bacterial cell wall is a crystal lattice structure formed from  linear chains of two alternating amino sugars, namely "N"-acetylglucosamine (GlcNAc or NAGA) and "N"-acetylmuramic acid (MurNAc or NAMA). The alternating sugars are connected by a β-(1,4)-glycosidic bond. Each MurNAc is attached to a short (4- to 5-residue) amino acid chain, containing -alanine, -glutamic acid, "meso"-diaminopimelic acid, and -alanine in the case of "Escherichia coli" (a Gram-negative bacterium) or -alanine, -glutamine, -lysine, and -alanine with a 5-glycine interbridge between tetrapeptides in the case of "Staphylococcus aureus" (a Gram-positive bacterium). Peptidoglycan is one of the most important sources of D-amino acids in nature. By enclosing the inner  membrane, the peptididoglycan layer protects the cell from lysis caused by the turgor pressure of the cell. When the cell wall grows, it retains its shape throughout its life, so a rod shape will remain a rod shape, and a spherical shape will remain a spherical shape for life. This happens because the freshly added septal material of synthesis transforms into a hemispherical call for the offspring cells. Cross-linking between amino acids in different linear amino sugar chains occurs with the help of the enzyme DD-transpeptidase and results in a 3-dimensional structure that is strong and rigid . The specific amino acid sequence and molecular structure vary with the bacterial species. Biosynthesis. The peptidoglycan monomers are synthesized in the cytosol and are then attached to a membrane carrier bactoprenol. Bactoprenol transports peptidoglycan monomers across the cell membrane where they are inserted into the existing peptidoglycan. In the first step of peptidoglycan synthesis, glutamine, which is an amino acid, donates an amino group to a sugar, fructose 6-phosphate. This turns fructose 6-phosphate into glucosamine-6-phosphate. In step two, an acetyl group is transferred from acetyl CoA to the amino group on the glucosamine-6-phosphate creating "N"-acetyl-glucosamine-6 -phosphate. In step three of the synthesis process, the "N"-acetyl-glucosamine-6-phosphate is isomerized, which will change "N"-acetyl-glucosamine-6-phosphate to "N"-acetyl-glucosamine-1-phosphate. In step 4, the "N"-acetyl-glucosamine-1-phosphate, which is now a monophosphate, attacks UTP. Uridine triphosphate, which is a pyrimidine nucleotide, has the ability to act as an energy source. In this particular reaction, after the monophosphate has attacked the UTP, an inorganic pyrophosphate is given off and is replaced by the monophosphate, creating UDP-N-acetylglucosamine (2,4). (When UDP is used as an energy source, it gives off an inorganic phosphate.) This initial stage, is  used to create the precursor for the NAG in peptidoglycan. In step 5, some of the UDP-N-acetylglucosamine (UDP-GlcNAc) is converted to UDP-MurNAc (UDP-N-acetylmuramic acid) by the addition of a lactyl group to the glucosamine. Also in this reaction, the C3 hydroxyl group will remove a phosphate from the alpha carbon of phosphoenolpyruvate. This creates what is called an enol derivative that will be reduced to a “lactyl moiety” by NADPH in step six. In step 7, the UDP–MurNAc is converted to UDP-MurNAc pentapeptide by the addition of five amino acids, usually including  the dipeptide -alanyl--alanine. Each of these reactions requires the energy source ATP. This is all referred to as Stage one. Stage two occurs in the cytoplasmic membrane. It is in the membrane where a lipid carrier called bactoprenol carries peptidoglycan precursors through the cell membrane. Bactoprenol will attack the UDP-MurNAc penta, creating a PP-MurNac penta, which is now a lipid. UDP-GlcNAc is then transported to MurNAc, creating Lipid-PP-MurNAc penta-GlcNAc, a disaccharide, also a precursor to peptidoglycan. How this molecule is transported through the membrane is still not understood. However, once it is there , it is added to the growing glycan chain. The next reaction is known as tranglycosylation. In the reaction, the hydroxyl group of the GlcNAc will attach to the MurNAc in the glycan, which will displace the lipid-PP from the glycan chain. The enzyme responsible for this is transglycosylase. Inhibition. Some antibacterial drugs such as penicillin interfere with the production of peptidoglycan by binding to bacterial enzymes known as penicillin-binding proteins or DD-transpeptidases. Penicillin-binding proteins form the bonds between oligopeptide crosslinks in peptidoglycan. For a bacterial cell to reproduce through binary fission, more than a million peptidoglycan  subunits (NAM-NAG+oligopeptide) must be attached to existing subunits. Mutations in genes coding for transpeptidases that lead to reduced interactions with an antibiotic are a significant source of emerging antibiotic resistance. Muraymycins are one subclass of nucleoside antibiotics which act as competitive inhibitors for the natural substrate UDP-MurNAc-pentapeptide (UM5A) of Phospho-N-acetylmuramoyl-pentapeptide translocase (MraY). Lysozyme, which is found in tears and constitutes part of the body's innate immune system exerts its antibacterial effect by breaking the β-(1,4)-glycosidic bonds in peptidoglycan (see above). Pseudopeptidoglycan (Pseudomurein). In some archaea, i.e. members of the Methanobacteriales and in the  genus Methanopyrus, pseudopeptidoglycan (pseudomurein) has been found. In pseudopeptidoglycan the sugar residues are β-(1,3) linked "N"-acetylglucosamine and "N"-acetyltalosaminuronic acid. This makes the cell walls of such archaea insensitive to lysozyme. The biosynthesis of pseudopeptidoglycan has been described.