Bacterial and Archaeal Cell Wall Structures: Comparative Analysis and Functional Differences

Lecture 5: Bacterial and Archaeal Cell Wall

Learning Objectives

• Compare and contrast gram-positive, gram-negative, and archaeal cell wall structures.

• Describe the functional differences in cell membranes and cell walls.

Bacterial Cell Walls

General Properties and Functions

Bacterial cell walls are essential for maintaining cell shape, rigidity, and protection against osmotic pressure. They are also a major target for antibiotics.

• Osmotic Pressure: High solute concentration in the cytoplasm (~30 psi) necessitates a cell wall to prevent lysis.

• Shape and Rigidity: The wall confers structural integrity and defines the cell's morphology.

• Antibiotic Target: Many antibiotics disrupt cell wall synthesis or integrity.

Classification Based on Cell Wall Structure

• Gram-positive bacteria: Thick peptidoglycan layer and one cytoplasmic membrane.

• Gram-negative bacteria: Thin peptidoglycan layer, two membranes (cytoplasmic and outer membrane containing LPS).

Bacterial Cell Walls: Peptidoglycan

Structure and Composition

Peptidoglycan is a rigid polysaccharide layer unique to Bacteria, providing strength to the cell wall.

• Composed of alternating repeats of two modified glucose residues: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in β-1,4 linkages.

• Includes amino acids: L-alanine, D-alanine, D-glutamic acid, and either lysine or diaminopimelic acid (DAP) for peptide linkages.

• Long glycan chains are cross-linked by short peptides to form a strong, mesh-like structure.

Peptidoglycan Cross-Linking

• Chains are connected by cross-links (peptide bonds) of amino acids; the degree of cross-linking determines wall rigidity.

• Gram-negative: Peptide bond between DAP and terminal D-alanine.

• Gram-positive: Short peptide interbridges (e.g., Staphylococcus has 5 glycine residues).

Destruction of Peptidoglycan

• Lysozyme: Enzyme that cleaves NAG-NAM bonds; found in bodily secretions.

• Penicillin and β-lactam antibiotics: Prevent cross-linking of peptidoglycan chains.

• Both mechanisms lead to osmotic lysis of the cell.

Chemical Diversity

• Over 100 distinct peptidoglycans exist, varying in cross-linking/interbridge structures, but NAG and NAM remain constant.

Gram-Positive Bacterial Cell Walls

Structural Features

• Up to 90% peptidoglycan; multiple layers stacked and cross-linked for strength.

• Contain teichoic acids covalently bonded to peptidoglycan, contributing to the negative charge of the cell surface.

• Teichoic acids bind Ca2+ and Mg2+ prior to transport.

• Lipoteichoic acids: Teichoic acids covalently bound to membrane lipids.

Gram-Negative Bacterial Cell Walls

Structural Features

• Outer membrane is a second lipid bilayer, containing lipopolysaccharide (LPS).

• LPS consists of:

  1. O-specific polysaccharide

  2. Core polysaccharide

  3. Lipid A (endotoxin, toxic component)

• LPS acts as an additional barrier against harmful agents, but does not provide structural strength.

• Braun lipoprotein attaches the outer membrane to the peptidoglycan.

• Three main layers: outer membrane (LPS), thin peptidoglycan, cytoplasmic membrane.

• The periplasm is the space between the membranes.

Periplasm and Porins

• Outer membrane is generally impermeable to proteins and large molecules; proteins remain in the periplasm.

• Periplasm contains hydrolytic enzymes, binding proteins, and chemoreceptors.

• Small molecules cross via porins (channel proteins); some are specific, others nonspecific.

• Porins are typically composed of three identical subunits, forming multiple pores.

Prokaryotes That Lack Cell Walls

Examples and Adaptations

• Mycoplasma (bacterial pathogens) and Thermoplasma (archaeal thermophiles) lack cell walls.

• They possess unusually tough cytoplasmic membranes or inhabit osmotically protected environments.

• May use sterols or lipoglycans for membrane strength and rigidity.

Archaeal Cell Walls

General Features

• No peptidoglycan or outer membrane; diverse wall chemistries.

Pseudomurein

• Found in some methanogenic Archaea; similar to peptidoglycan but with key differences.

• Backbone: repeating subunits of N-acetylglucosamine and N-acetyltalosaminuronic acid (replaces NAM).

• Contains β-1,3 glycosidic bonds (not β-1,4); all amino acids are L-stereoisomers.

• Resistant to lysozyme and penicillin.

Other Archaeal Cell Wall Types

• Some Archaea lack pseudomurein and have other polysaccharide polymers.

• Most common type: paracrystalline surface layer (S-layer).

• S-layer: interlocking protein or glycoprotein molecules forming paracrystalline structures (hexagonal, tetragonal, trimeric symmetries).

• In some, S-layer is the sole wall component; in others, it is combined with polysaccharides.

• S-layer is always the outermost layer, allowing small molecules to pass while excluding large ones.

Comparative Table: Bacterial vs. Archaeal Cell Walls

Feature	Gram-Positive Bacteria	Gram-Negative Bacteria	Archaea
Main Wall Polymer	Peptidoglycan (thick)	Peptidoglycan (thin)	Pseudomurein or S-layer
Membranes	1 (cytoplasmic)	2 (cytoplasmic + outer)	1 (cytoplasmic)
Additional Components	Teichoic acids	LPS, Braun lipoprotein, porins	Glycoproteins, polysaccharides
Lysozyme Sensitivity	Yes	Yes	No (if pseudomurein)
Antibiotic Sensitivity	Yes (β-lactams)	Yes (β-lactams)	No (if pseudomurein)

Summary

• Bacterial and archaeal cell walls are structurally and chemically distinct, reflecting evolutionary adaptations to environmental pressures.

• Understanding these differences is crucial for microbiology, especially in the context of antibiotic development and microbial physiology.