Bacterial Cell Wall

Abstract Most bacteria are encased in walls that protect the cells against lysis by osmotic forces from within and from chemical or biological assaults from outside. These walls are assembled in layers consisting of four principal components: inner membrane, peptidoglycan, outer membrane (OM) and S‐layer. The first two are the basic constituents of bacterial walls, the OM is a hallmark of one branch of bacteria and S‐layers are optional in many different microorganisms. The two fundamental types of wall are Gram‐type positive, which have no OM, and Gram‐type negative, which have an OM. Bacteria also produce various secondary polymers, such as teichoic or mycolic acids, that associate with peptidoglycan and alter its chemical characteristics. Finally, and in addition to its protective role, the wall imparts to bacterial cells their specific shapes and is organised to facilitate the transport of vital chemicals into and out of the cell. Key Concepts The cell wall protects bacteria from lysis, chemical assault and attack by the immune system. The bacterial cell wall consists of an inner (plasma) membrane, a rigid peptidoglycan exoskeleton and, in some cases, an outer membrane and/or an S‐layer. Peptidoglycan is composed of a long chain of repeating disaccharides linked to one another by short peptide side chains, which creates a single macromolecule surrounding the bacterial cell. The biological properties of many bacterial cell walls are strengthened or enhanced by the addition of secondary cell wall glycopolymers. The bacterial world is divided into two main groups: those that have a single membrane (the plasma or inner membrane) and those that have two membranes (inner and outer membranes). S‐layers are paracrystalline arrays of a single protein that completely cover the exterior of many bacteria. Bacterial shape is chiefly determined by cytoskeletal proteins that direct the synthesis of the overall structure of peptidoglycan.