L3 Microbial Cell Structure and Function: Cell Walls, Membranes, and Surface Structures

Microbial Cell Structure and Function

Overview

This chapter explores the structural diversity of microbial cells, focusing on the composition and function of cell walls, membranes, and surface structures in Archaea, Gram-negative bacteria, and other microorganisms. Understanding these features is essential for appreciating microbial physiology, taxonomy, and interactions with the environment.

Archaeal Cell Walls

General Features

• Peptidoglycan Absence: Archaeal cell walls do not contain peptidoglycan, which is a hallmark of bacterial cell walls.

• Outer Membrane: Archaea typically lack an outer membrane.

• Pseudomurein: Some methanogenic Archaea possess pseudomurein, a polysaccharide similar to peptidoglycan.

• Composition: Pseudomurein is composed of N-acetylglucosamine and N-acetyltalosaminuronic acid.

• S-layers: Many Archaea have S-layers (surface-layer proteins) as the most common cell wall type.

Pseudomurein Structure

• Polysaccharide Backbone: Alternating units of N-acetylglucosamine (G) and N-acetyltalosaminuronic acid (T).

• Glycosidic Linkage: β(1,3) linkage, which is lysozyme-insensitive (unlike the β(1,4) linkage in bacterial peptidoglycan).

• Peptide Cross-links: Peptides such as L-Glu, L-Ala, L-Lys connect the polysaccharide chains.

Figure 2.11: Diagram of pseudomurein structure, highlighting the chemical differences from bacterial peptidoglycan.

S-layers

• Definition: S-layers are crystalline arrays of protein or glycoprotein that form the outermost cell envelope in many Archaea.

• Function: Provide structural support, protection, and can act as molecular sieves.

Figure 2.14: Electron micrograph showing the regular, lattice-like structure of an archaeal S-layer.

LPS: The Outer Membrane in Gram-Negative Bacteria

General Features

• Peptidoglycan Content: Gram-negative bacterial cell walls contain approximately 10% peptidoglycan.

• Outer Membrane: The majority of the cell envelope is composed of an outer membrane, also known as the lipopolysaccharide (LPS) layer.

LPS Structure and Function

• Components: LPS consists of a core polysaccharide and an O-polysaccharide (O-antigen).

• Phospholipid Replacement: LPS replaces most phospholipids in the outer half of the outer membrane.

• Endotoxin: The toxic component of LPS, responsible for the symptoms of Gram-negative bacterial infections.

Component	Location	Function
Peptidoglycan	Periplasmic space	Structural support
LPS (core + O-antigen)	Outer membrane	Protection, immune evasion, endotoxin activity
Phospholipids	Inner leaflet of outer membrane	Membrane fluidity

Figure 2.12: Diagram showing the arrangement of LPS and phospholipids in the Gram-negative outer membrane.

Key Terms

• Peptidoglycan: A polymer of sugars and amino acids forming a mesh-like layer outside the plasma membrane of most bacteria.

• Pseudomurein: A peptidoglycan-like polymer found in some Archaea.

• S-layer: A regularly structured layer composed of protein or glycoprotein.

• LPS (Lipopolysaccharide): A large molecule consisting of a lipid and a polysaccharide, forming the outer leaflet of the outer membrane in Gram-negative bacteria.

• Endotoxin: The lipid A component of LPS, which is toxic to animals.

Additional info:

• Archaeal cell walls are highly variable and can include polysaccharides, proteins, or glycoproteins, depending on the species.

• Gram-positive bacteria lack an outer membrane and have a much thicker peptidoglycan layer compared to Gram-negative bacteria.

• The β(1,3) glycosidic bond in pseudomurein is not cleaved by lysozyme, making Archaea resistant to this enzyme.