Lesson 10 - Bacterial Cell Envelope

What are the three major bacterial cell envelope types?

• Gram-positive bacteria: Thick cell wall.

• Gram-negative bacteria: Thin cell wall.

• Mycobacteria: Complex, multilayered cell wall.

• These additional envelope layers provide structural support and protection.

What is peptidoglycan, and how does it form the bacterial cell wall?

• Peptidoglycan (murein) makes up most bacterial cell walls, also called the sacculus.

• It consists of sugar chains connected by cross-bridges.

• The interconnected network forms one enormous molecule surrounding the cell.

• The cell wall is a single molecule

• It provides structural support and protection.

What is the structure of bacterial peptidoglycan?

• Consists of alternating NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid) sugars.

• NAM is attached to short peptides containing 4–6 amino acids.

• Peptides cross-link parallel glycan strands, including through bonds involving D-Alanine and m-Diaminopimelic acid

• These connections create a strong, interconnected cell wall.

What are the functions of peptidoglycan, and how do penicillin and vancomycin interfere with its formation?

• Functions:

  • Maintains bacterial cell shape.

  • Helps withstand turgor pressure from the cytoplasm. (osmotic pressure
    arising from the cytoplasm and pressing against the cytoplasmic membrane)

• Antibiotics:

  • Penicillin inhibits transpeptidase, preventing peptide cross-linking.

  • Vancomycin binds D-Ala–D-Ala, preventing cross-bridge formation.

• Peptidoglycan is an excellent antibiotic target because it is unique to bacteria.

Where can bacteria insert new peptidoglycan during cell wall growth?

• Different bacterial species synthesize new peptidoglycan at:

  • Dispersed zones throughout the cell wall.

  • The septum (division site) only.

  • The cell poles (ends of the cell).

What characterizes the cell wall of Gram-positive bacteria?

• Thick peptidoglycan layer.

• Typically 20–80 nm thick.

• Consists of many stacked layers, up to 40 in some species.

What are teichoic acids, and how do wall teichoic acids differ from lipoteichoic acids?

• Teichoic acids are polymers of glycerol or ribitol (sugars) linked by phosphodiester bonds.

• They may be decorated with sugars or amino acids.

• Found throughout the thick peptidoglycan wall of Gram-positive bacteria.

• Either…

  • Wall teichoic acids (WTA): Linked to peptidoglycan.

  • Lipoteichoic acids (LTA): Anchored to membrane phospholipids.

• Memory trick: Wall = wall; Lipo = lipid.

What are the advantages and disadvantages of Gram-positive bacterial cell envelopes?

• Advantages:

  • Thick peptidoglycan wall reinforced by teichoic acids provides strength.

  • Protects against osmotic lysis.

  • Stains purple during Gram staining.

• Disadvantages:

  • Exposed cell wall is susceptible to lysozyme and other wall-targeting substances.

  • Generally more susceptible to antibiotics than Gram-negative bacteria.

  • Examples: Bacillus cereus (rods), Streptococcus pyogenes (chains), Staphylococcus aureus (clusters).

What are the main structural features of Gram-negative bacterial cell envelopes?

• Diderm: two membranes separated by the periplasm (space between two membranes).

• Thin peptidoglycan layer (1–2 sheets) in the periplasm.

• Outer membrane contains…

  • LPS (lipopolysaccharides) bridged with cations (e.g., Mg2+). Formidable barrier

  • Transmembrane proteins called porins (act as a pore, through which molecules can diffuse)

  • Lipoproteins help anchor peptidoglycan in place.

• Inner membrane contains proteins with functions distinct from outer membrane proteins.

What are the three regions of Gram-negative lipopolysaccharide (LPS)?

• Reminder: All Gram-negative bacteria have an outer membrane with LPS, major permeability barrier

• O-antigen: Repeating polysaccharide; composition varies between strains.

• Core polysaccharide: Relatively conserved carbohydrate region.

• Lipid A: Anchors LPS in the outer membrane

• LPS act as endotoxin = molecules that are harmless when pathogen is intact, but become toxic and activate immune response when it is released from a lysed cell

• LPS also contributes to the outer membrane permeability barrier

What is serotyping, and how is LPS involved?

• LPS is one of most structurally variable macromolecules on Earth

• Serotyping distinguishes bacteria based on differences in surface antigens.

• Different compositions of LPS among the same species of pathogen = different serovars of the same pathogen

• Example: E. coli O157:H7.

• O157 identifies the O-antigen type; H7 identifies the flagellar antigen type.

Species vs Serovar

• Species: What type of bacterium is it? (E. coli)

• Serovar: Which antigenic variant of that species is it? (E. coli O157:H7)

  • Imagine we have three bacteria that all belong to the species E. coli.

  • Even though they're the same species, their LPS structures can differ, particularly in the O-antigen portion.

What are the advantages and disadvantages of Gram-negative bacterial cell envelopes?

Advantages:

• Outer membrane acts as an excellent selective permeability barrier.

• Protects against a wide range of toxic molecules.

• Protects the thin peptidoglycan layer.

• Gram-negative bacteria stain pinkish-red.

Disadvantages:

• Outer membrane is energetically expensive to build and maintain.

• Usually have larger genomes containing genes for outer membrane components.

Review: Hopanoids

• Membranes also include planar molecules that fill gaps between hydrocarbon chains

• In eukaryotic membranes, the reinforcing agents are sterols, such as cholesterol

• In bacteria, the same function is served by hopanoids, or hopanes

• Prokaryotes regulate membrane fluidity and rigidity by varying the abundance of these different membrane lipids and hopanoids