Peptidoglycan

Lore & Background

Peptidoglycan is composed of linear chains of two alternating amino sugars, N-acetylglucosamine (GlcNAc or NAG) and N-acetylmuramic acid (MurNAc or NAM), connected by β-(1,4)-glycosidic bonds. Each MurNAc is attached to a short amino acid chain of 4 to 5 residues, which varies by bacterial species—for example, in Escherichia coli (gram-negative) the chain contains L-alanine, D-glutamic acid, meso-diaminopimelic acid, and D-alanine; in Staphylococcus aureus (gram-positive) it contains L-alanine, D-glutamine, L-lysine, and D-alanine with a 5-glycine interbridge. Cross-linking between amino acids in different chains occurs via the enzyme DD-transpeptidase, resulting in a strong, rigid 3-dimensional structure. 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, peptidoglycan forms around 40 to 90% of the cell wall's dry weight in gram-positive strains but only about 10% in gram-negative strains. The presence of high levels of peptidoglycan is the primary determinant of gram-positive characterization. Peptidoglycan is involved in binary fission during bacterial cell reproduction. L-form bacteria and mycoplasmas, both lacking peptidoglycan cell walls, do not proliferate by binary fission but by a budding mechanism. The invention of rigid peptidoglycan cell walls in bacteria was probably the prerequisite for their survival, extensive radiation, and colonization of virtually all habitats of the geosphere and hydrosphere.

Reader's Guide

Peptidoglycan is fundamental to bacterial biology and medicine. Its unique structure—a mesh-like layer of sugars and amino acids—provides the mechanical strength that allows bacteria to withstand osmotic pressure and maintain cell shape. The difference in peptidoglycan thickness between gram-positive and gram-negative bacteria is the basis of the Gram staining technique, a cornerstone of clinical microbiology for bacterial identification. Peptidoglycan is also the target of major antibiotics, such as penicillin, which inhibits the DD-transpeptidase (penicillin-binding protein) involved in cross-linking. The biosynthesis pathway, involving stages in the cytosol and cytoplasmic membrane with enzymes like MurA, MurB, and flippase MurJ, offers multiple targets for antimicrobial development. Additionally, peptidoglycan recognition by the immune system is evolutionarily conserved, and modifications to its structure can affect immune responses. The absence of peptidoglycan in archaea (which have pseudopeptidoglycan in some groups) and in L-form bacteria and mycoplasmas highlights its essential role in typical bacterial cell division and survival.

Did You Know?

• Peptidoglycan is one of the most important sources of D-amino acids in nature.
• Archaea do not contain peptidoglycan; some contain pseudopeptidoglycan (pseudomurein) with β-(1,3) linked sugars.
• Particles of approximately 2 nm can pass through the peptidoglycan in both gram-positive and gram-negative bacteria.