Microbial Cell Wall and Membrane: Structure, Function, and Diversity

Microbial Cell Wall and Membrane

Course Learning Outcomes

This section outlines the key objectives for understanding microbial cell walls and membranes, focusing on their structure, function, and diversity among different domains of life.

• Structure and function of the cytoplasmic membrane

• Similarities and differences between bacterial and archaeal membranes

• Structure and function of the cell wall

• Differences between Gram-positive and Gram-negative bacteria

• Gram stain procedure and its relation to cell wall structure

• Peptidoglycan synthesis

• Acid-fast bacteria and cell envelope diversity

• Borrelia burgdorferi: cell wall, disease, diagnosis, and treatment

Cytoplasmic Membrane

Structure of the Cytoplasmic Membrane

The cytoplasmic membrane is a fundamental component of all microbial cells, acting as a selective barrier and interface with the environment.

• Phospholipid bilayer: Composed of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Integral (transmembrane) proteins span the membrane, while peripheral proteins are attached to the surface.

• Function: Maintains cellular integrity, controls transport, and hosts metabolic activities.

Example: The fluid mosaic model describes the dynamic nature of the membrane, with proteins and lipids able to move laterally within the layer.

Membrane Composition

Membrane composition varies between Bacteria, Archaea, and Eukarya, influencing stability and function.

• Bacterial membranes: Phospholipids with fatty acid chains linked to glycerol via ester bonds.

• Archaeal membranes: Phospholipids with isoprenoid chains linked to glycerol via ether bonds, which are more chemically stable.

• Glycerophospholipids: Main component, consisting of glycerol, phosphate, and fatty acids.

Example: Archaeal membranes may form monolayers or bilayers, contributing to their ability to survive extreme environments.

Membrane Architecture in Archaea

Unique Features of Archaeal Membranes

Archaeal membranes are structurally distinct from those of Bacteria and Eukarya, providing resilience in harsh conditions.

• Ether linkages: Connect glycerol to isoprenoid chains, increasing chemical stability.

• Isoprenoid chains: Branched hydrocarbon chains derived from isoprene units.

• Monolayer formation: Some Archaea have lipid monolayers (e.g., crenarchaeol) for added stability at high temperatures.

• Glycerol diether and tetraether: Variations in membrane lipid structure among Archaea.

Example: Thermophilic Archaea often possess tetraether lipids, allowing survival in boiling hot springs.

Functions of the Cytoplasmic Membrane

Key Functions

The cytoplasmic membrane is essential for maintaining homeostasis and mediating interactions with the environment.

• Selective permeability: Regulates entry and exit of substances.

• Energy generation: Site of electron transport and ATP synthesis in prokaryotes.

• Signal transduction: Hosts receptors and proteins for environmental sensing.

Transport Across Membranes

Mechanisms of Transport

Microbial cells use various mechanisms to move substances across the cytoplasmic membrane.

• Passive transport: Movement down a concentration gradient (diffusion, facilitated diffusion).

• Active transport: Movement against a gradient, requiring energy (primary and secondary transport).

• Group translocation: Substance is chemically modified during transport (e.g., phosphotransferase system for glucose).

Example: Facilitated diffusion uses membrane proteins to transport molecules like glucose without energy input.

Cell Wall Structure and Function

Role of the Cell Wall

The cell wall provides structural support and protection against osmotic stress.

• Prevents osmotic lysis: Maintains cell shape and integrity in hypotonic environments.

• Composed of peptidoglycan (in Bacteria): A mesh-like polymer of sugars and amino acids.

• Target for antibiotics: Unique to bacteria, making it a prime target for antimicrobial drugs.

Peptidoglycan Structure

Peptidoglycan is the main component of bacterial cell walls, providing strength and rigidity.

• Subunits: Alternating N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG).

• Glycosidic bonds: Link NAM and NAG in chains.

• Peptide cross-links: Connect chains, forming a strong, flexible network.

Example: Lysozyme breaks β-1,4 glycosidic bonds in peptidoglycan, leading to cell lysis.

Gram-Positive vs. Gram-Negative Cell Walls

Structural Differences

Gram staining differentiates bacteria based on cell wall structure, which affects staining properties and antibiotic susceptibility.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick, multi-layered	Thin, single-layered
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains LPS)
Lipopolysaccharide (LPS)	Absent	Present
Stain Color	Purple	Pink/Red

Gram Stain Procedure

The Gram stain is a differential staining technique used to classify bacteria.

1. Crystal violet (primary stain)

2. Iodine (mordant)

3. Alcohol (decolorizer)

4. Safranin (counterstain)

Result: Gram-positive bacteria retain crystal violet and appear purple; Gram-negative bacteria lose it and take up safranin, appearing pink.

Special Cell Walls: Acid-Fast Bacteria

Acid-Fast Cell Walls

Acid-fast bacteria, such as Mycobacterium species, have unique cell walls with waxy lipids (mycolic acids).

• Waxy lipid layer: Provides resistance to desiccation and antibiotics.

• Staining: Acid-fast stain uses heat and carbol fuchsin to penetrate the waxy layer.

• Clinical relevance: Includes pathogens like Mycobacterium tuberculosis.

Cell Envelope Diversity

Archaeal Cell Walls

Archaeal cell walls lack peptidoglycan and may contain pseudomurein, polysaccharides, proteins, or glycoproteins.

• Pseudomurein: Similar to peptidoglycan but with different sugar linkages.

• Surface layers (S-layers): Protein or glycoprotein layers providing structural support.

Microbial Diseases Related to Cell Wall Structure

Streptococcus pyogenes

Streptococcus pyogenes is the causative agent of strep throat and other infections.

• Virulence factors: M protein, exotoxins, and capsule.

• Diseases: Strep throat, impetigo, scarlet fever, and toxic shock syndrome.

• Complications: Untreated infections can lead to rheumatic fever due to molecular mimicry.

Streptococcus pneumoniae

Streptococcus pneumoniae causes pneumonia and other respiratory infections.

• Capsule: Major virulence factor, protects against phagocytosis.

• Drug resistance: Some strains are resistant to antibiotics, complicating treatment.

Borrelia burgdorferi

Borrelia burgdorferi is the causative agent of Lyme disease.

• Cell wall features: Lacks typical peptidoglycan structure, contributing to immune evasion and chronic symptoms.

• Diagnosis and treatment: Based on clinical symptoms, serology, and antibiotic therapy.

Example: Chronic Lyme disease symptoms may be linked to persistent infection and immune response to unique cell envelope components.

Summary Table: Bacterial Cell Wall Types

Type	Main Component	Special Features	Example Organisms
Gram-Positive	Thick peptidoglycan	Teichoic acids, no outer membrane	Streptococcus pyogenes
Gram-Negative	Thin peptidoglycan, outer membrane	LPS, porins	Escherichia coli
Acid-Fast	Peptidoglycan, mycolic acids	Waxy cell wall, resistant to staining	Mycobacterium tuberculosis
Archaeal	Pseudomurein, S-layer	No peptidoglycan, ether-linked lipids	Halobacterium

Key Equations and Chemical Structures

• Peptidoglycan glycosidic bond: $\text{NAM} - \beta(1,4) - \text{NAG}$

• Phospholipid structure: $\text{Glycerol} + 2 \times \text{Fatty Acids} + \text{Phosphate}$

• Archaeal ether linkage: $\text{Glycerol} - \text{O} - \text{Isoprenoid Chain}$

Additional info: Some details on membrane protein function, antibiotic resistance, and clinical relevance were expanded for academic completeness.