Microbiology: The Structure and Function of Bacterial Cell Walls

Role and Importance of the Cell Wall

• The cell wall is a critical topic in microbiology and serves as the primary protector of the organism.
• The cell wall is located on the outside of the organism and provides two essential physical properties:
  • Rigidity: It maintains the structural integrity of the cell.
  • Shape: While prokaryotic cells come in various shapes, these shapes ultimately arise from how the outer shell (the wall) is constructed.
• General Structure: In an idealized bacterial cell, the wall is positioned outside the cell membrane (plasma membrane).
  • In some contexts, the cell membrane is considered part of the cell wall system, though they are distinct layers.
  • The wall must be both strong for protection and flexible enough to meet the demands of the cell.
• Osmotic Protection: A key role of the cell wall is protecting the cell from osmotic stress, which results from the tonicity of the environment.

Osmotic Stress and Tonicity

• Definition of Tonicity: A description of how cells gain or lose water based on the concentration of dissolved particles outside the cell versus inside the cell per unit of volume.
• Mechanism of Water Flow: Water seeks to dilute concentrations. It will flow toward the side with the higher concentration of dissolved particles.
• Scenarios of Tonicity (Red Blood Cell Example):
  • Hypertonic: The concentration of dissolved particles (e.g., salt water) is higher outside the cell than inside. Water flows net out of the cell, causing the cell to lose shape, dehydrate, and flatten.
  • Isotonic: Dissolved particles are equal on both sides. Water flows in and out at equal rates; there is no net flow, and the cell remains stable.
  • Hypotonic: Dissolved particles are at a higher concentration inside the cell than outside (e.g., pure water). Water rushes into the cell, which can lead to bursting (osmotic lysis).
• Tonicity in Prokaryotic Cells:
  • Most bacteria live in environments where there are more dissolved particles inside the cell than outside (hypotonic environments).
  • The cell wall prevents the cell from expanding past a certain point, providing mechanical protection against bursting.
  • In a hypertonic solution, the cell contents will still shrink, but they will shrink away from the interior of the cell wall.
• Semi-permeability: Despite its protective strength, the cell wall must be semi-permeable to allow materials to move in and out, and it offers some protection against toxins.

Comparative Structure of Cell Walls

• Plants and Algae: Use cell walls made of cellulose, which are polymers of glucose. These involve hydrogen bonding between different polymer chains to provide strength (e.g., the strength found in wood).
• Bacteria: The primary strength-providing component is peptidoglycan. Bacteria are categorized into two major arrangements based on the construction of these walls:
  • Gram-positive: Features a thick peptidoglycan layer located outside of the single cell membrane.
  • Gram-negative: Features a thinner peptidoglycan layer located in the space between two membranes (the plasma membrane and an outer membrane).
• Archaebacteria (Archaea):
  • They do not have peptidoglycan.
  • Many use proteins or glycoproteins for protection.
  • They are characterized by linked glycerol to long, branched lipids.
  • Some Archaea lack a cell wall entirely.

Chemical Structure of Peptidoglycan

• Etymology:
  • Peptido/Pepta: Refers to peptides (amino acid chains).
  • Glycan: Refers to polysaccharides (sugars).
• Chemical Components: Peptidoglycan is composed of alternating subunits of two sugars:
  • NAG: $N\text{-acetylglucosamine}$
  • NAM: $N\text{-acetylmuramic acid}$
• Subunit Details:
  • NAM: Contains an attached tetrapaptide (a chain of $4$ amino acids) and a lactic acid group. This peptide chain is the site for cross-linking.
  • NAG: Contains an acetyl group but lacks the peptide side chains found on NAM.
• Cross-linking: Strength is derived from cross-linking the tetrapeptides of a NAM on one polymer strand to the tetrapeptides of a NAM on an adjacent polymer strand.
  • Individual strands are like spaghetti; they are weak alone but very strong when tied together in sheets.
  • This cross-linking provides tensile strength and flexibility.

Detailed Anatomy of Gram-Positive Cell Walls

• Peptidoglycan Layer: Highly prominent, making up to $90\%$ of the total cell wall mass.
  • It can consist of up to $30$ layers of peptidoglycan.
• Teichoic Acids: Acidic polysaccharides with a negative charge that make up to $10\%$ of the wall mass.
  • Teichoic acid: Polysaccharides attached specifically to the peptidoglycan layer (connected to the amino acids on the NAM subunits or other sugars), acting like structural pillars.
  • Lipoteichoic acid: Teichoic acids that are anchored into the lipid bilayer of the plasma membrane. They span the entire peptidoglycan layer and emerge at the surface.
• Functions: These acids provide structural support, rigidity, and play a role in the formation of biofilms (e.g., the slimy layer of bacteria on teeth in the morning).

Detailed Anatomy of Gram-Negative Cell Walls

• Complex Layering: More complex than Gram-positive, featuring a double membrane system.
• Peptidoglycan: Much thinner than in Gram-positive cells, representing only $5$ to $20\%$ of the cell wall.
• Outer Membrane: A lipid bilayer located outside the peptidoglycan.
  • The inner leaflet consists of phospholipids.
  • The outer leaflet contains Lipopolysaccharides (LPS).
  • The outer membrane contains porins, which are protein channels that allow the passage of specific materials.
• Periplasmic Space: The gap between the plasma membrane and the outer membrane.
  • It contains the thin peptidoglycan layer.
  • It is filled with periplasm, a gel-like matrix (high water content).
  • It is a site of high enzymatic activity, such as the hydrolysis of disaccharides into monosaccharides.

Lipopolysaccharide (LPS) Structure and Function

• LPS is unique to Gram-negative bacteria and is divided into three sections:
  1. Lipid A: The portion anchored in the outer membrane. It acts as an endotoxin. If the bacteria die and break apart, Lipid A is released and can be toxic to the host.
  2. Core Polysaccharide: The middle portion that is relatively similar across different bacterial species.
  3. O-specific Polysaccharide (O-antigen): The outermost portion consisting of various arrangements of sugars.
     • It acts as an antigen that antibodies can recognize.
     • It is used in serology to identify specific bacterial strains.
     • Example: In the strain $E.\,coli\,O157:H7$, the "O" refers to the specific pattern of this O-specific polysaccharide.

Mechanisms of Cell Wall Destruction

• Organisms and medical treatments often target the cell wall to kill bacteria.
• Lysozyme:
  • An enzyme found in human tears, milk, and saliva as a first line of defense.
  • Mechanism: It catalyzes a hydrolysis reaction ($H_2O$ is added to the covalent bond) to cleave the polymer bond between NAG and NAM.
  • Note: Do not confuse lysozyme (the enzyme) with the lysosome (the cellular organelle).
• Penicillin:
  • A major antibiotic that targets the development of the peptidoglycan layer.
  • Mechanism: It binds to Penicillin Binding Proteins (PBPs).
  • PBPs (such as transpeptidase) are responsible for building the peptide cross-links between polymer strands.
  • By inhibiting PBPs, penicillin prevents cross-linking, weakening the cell wall until osmotic pressure causes the cell to burst.

Synthesis of Peptidoglycan and Cellular Structure Review

• Internal Structures: Internal to the cell membrane are the cytoplasm, ribosomes, a nuclear region, and occasionally granules or vesicles.
• External Structures: Depending on the species, bacteria may have a capsule, flagella, or pili.
• Stages of Peptidoglycan Synthesis:
  1. Five amino acids are added to $N\text{-acetylmuramic acid}$ (NAM).
  2. $N\text{-acetylglucosamine}$ (NAG) is added to the NAM to form a peptidoglycan precursor.
  3. The precursor is transported across the cell membrane to a cell wall acceptor in the periplasm.
  4. Precursors bind to acceptors and undergo extensive cross-linking via enzymes like transpeptidase and $D\text{-alanyl carboxypeptidase}$ (the Penicillin Binding Proteins).
• The final result is multiple layers of cross-linked peptidoglycan forming a robust cell wall.