Bacterial Cell Wall and Envelope
The bacterial cell envelope is the layered boundary, plasma membrane, cell wall, and in Gram-negative bacteria an additional outer membrane, that surrounds the cytoplasm. Its central structural element is peptidoglycan, a mesh-like polymer that gives the cell its shape and protects it against osmotic lysis, and the way the envelope is organized defines the principal structural division of bacteria.
Definition
The bacterial cell wall and envelope is the multilayered surface structure of the bacterial cell, comprising the plasma membrane and an external peptidoglycan-based wall (with an additional outer membrane in Gram-negative bacteria), that maintains cell shape and resists internal turgor pressure.
Scope
This topic covers the chemistry and architecture of peptidoglycan, the Gram-positive envelope (a thick wall with teichoic acids), the Gram-negative envelope (a thin wall enclosed by an outer membrane bearing lipopolysaccharide), and the functional roles of the envelope in shape, integrity, and permeability. It treats the staining procedure that reveals this distinction in a sibling topic.
Core questions
- How does peptidoglycan architecture confer shape and osmotic protection?
- What structural differences distinguish the Gram-positive from the Gram-negative envelope?
- How does the outer membrane shape the permeability of Gram-negative bacteria?
Key concepts
- Peptidoglycan (murein) sacculus
- Glycan strands cross-linked by peptide bridges
- Gram-positive thick wall with teichoic acids
- Gram-negative thin wall and outer membrane
- Lipopolysaccharide and the outer membrane
- Periplasmic space
- Osmotic protection and turgor
Mechanisms
Peptidoglycan is built from glycan chains of alternating N-acetylglucosamine and N-acetylmuramic acid that are cross-linked by short peptides into a single, mesh-like macromolecule, the sacculus, that encloses the cell and counteracts turgor pressure. In Gram-positive bacteria this wall is thick and threaded with teichoic and lipoteichoic acids; in Gram-negative bacteria the peptidoglycan layer is thin and lies in a periplasmic space between the plasma membrane and an asymmetric outer membrane whose outer leaflet is composed of lipopolysaccharide. The outer membrane acts as a permeability barrier with channel proteins controlling the entry of solutes. These envelope differences underlie the differential retention of dye in the Gram stain and many of the structural distinctions among bacteria.
Clinical relevance
The envelope is the structural feature that separates Gram-positive from Gram-negative bacteria and contributes to intrinsic differences in barrier properties between the two groups; lipopolysaccharide of the Gram-negative outer membrane is an important surface molecule recognized by the host. This entry describes envelope structure as reference material and is not a basis for individual diagnostic or treatment decisions.
Evidence & guidelines
The molecular structure of peptidoglycan and the architecture of the Gram-positive and Gram-negative envelopes are established in the biochemical and cell-biological literature and synthesized in standard microbiology textbooks.
History
The wall was recognized early as the structure responsible for bacterial shape and for the Gram reaction, and the elucidation of peptidoglycan chemistry in the mid-twentieth century, followed by detailed models of its architecture, explained how a single covalent network can both shape and protect the cell.
Key figures
- Waldemar Vollmer
- Thomas Silhavy
- Miguel de Pedro
Related topics
Seminal works
- vollmer-2008
- silhavy-2010
Frequently asked questions
- What is peptidoglycan?
- Peptidoglycan (murein) is a mesh-like polymer of glycan chains cross-linked by peptides that forms the bacterial cell wall, giving the cell its shape and protecting it against osmotic lysis.
- How do Gram-positive and Gram-negative envelopes differ?
- Gram-positive bacteria have a thick peptidoglycan wall with teichoic acids and no outer membrane, whereas Gram-negative bacteria have a thin peptidoglycan layer enclosed by an outer membrane that contains lipopolysaccharide.