Bacterial Cell Envelope: Walls and Membranes

• The cell envelope includes all materials enclosing the cytoplasm of bacteria.
• It has several layers including the cell wall and cytoplasmic membrane.
• Cytoplasmic membrane structure is similar across cell types, but the cell wall differs.

Cytoplasmic Membrane

• It is a phospholipid bilayer with integrated proteins.
• Proteins include enzymes for permeation, cell wall biosynthesis, transfer, signal transduction, and respiration.
• Polar phosphate groups are on the bilayer's exterior, nonpolar lipid chains on the interior.
• It serves as a permeability barrier, controlling molecule entry and exit.

Bacterial Cell Wall

• It protects against desiccation, toxins, and immune responses.
• It facilitates communication with the environment and other cells.
• Peptidoglycan (murein) is the main structural element.
• Peptidoglycan is a net-like polymer surrounding the cell, cross-linked by peptides.
• The peptidoglycan layer determines cell shape.
• The glycan part of peptidoglycan has alternating NAG and NAM monosaccharide subunits.
• The peptide portion is a short amino acid chain cross-linking polysaccharide strands.
• Gram-positive and gram-negative bacteria are differentiated by peptidoglycan quantity.

Gram-Negative vs. Gram-Positive Identification

• Gram staining is used to differentiate bacteria.
• Cell wall composition informs about pathogenicity and treatment options.

Gram-Negative Bacteria

• They have a complex cell wall with two membranes separated by the periplasmic space, which contains the peptidoglycan.
• They have a thin peptidoglycan layer (about 2 nm thick), making up ~10% of cell mass.
• They have an outer membrane containing lipopolysaccharide (LPS) and proteins.

Gram-Positive Bacteria

• They have thick, multi-layered peptidoglycan cell walls outside the cytoplasmic membrane.
• Their peptidoglycan layer is thick (15-80 nm) with around 40 layers.
• Peptidoglycan comprises ~30% of the cell wall's dry mass.
• Their peptidoglycan is covalently linked to teichoic acid, a polymer of glycerol units linked by phosphodiester bonds.

Peptidoglycan Composition

• It has a glycan portion and a peptide portion.
• The glycan portion's backbone consists of N-acetylglucosamine and N-acetylmuramic acid.
• These two sugars are bound by a 1,4-glycosidic bond.
• The carboxyl group of N-acetylmuramic acid connects to a tetrapeptide side chain.
• Adjacent tetrapeptide side chains are linked by a peptide cross-link.
• In gram-negative bacteria, the cross-link forms directly between diaminopimelic acid and D-alanine.
• In gram-positive bacteria, the cross-link forms between D-alanine and L-lysine via five glycine amino acids.

Gram-Positive Cell Walls

• They are composed of 90% peptidoglycan.
• The remaining 10% includes proteins, polysaccharides, and teichoic acid.
• Teichoic acid connects to either N-acetylmuramic acid or lipids in the cytoplasmic membrane.
• Teichoic acid bound to lipids is lipoteichoic acid (LTA).
• Teichoic acids covalently bound to peptidoglycan are wall teichoic acid (WTA).
• Teichoic acids provide rigidity and regulate autolysins.
• Teichoic acids are only in gram-positive bacteria.

Gram-Negative Cell Walls

• They have a more complex structure than gram-positive cell walls.
• An outer membrane exists external to a thin, single-layer peptidoglycan.
• The space between the outer and inner plasma membranes is the periplasmic space.
• Periplasmic space is smaller in gram-positive bacteria.
• The periplasm contains enzymes for degradation, transport, and motility.
• The outer membrane only exists in gram-negative bacteria.
• The outer membrane has proteins, phospholipids, and lipopolysaccharides (LPS).

Gram Stain Results

• Gram-positive cells have a thick peptidoglycan layer, retaining crystal violet dye.
• Gram-positive cells contain teichoic acids in their cell walls.
• Gram-positive cells appear purple after Gram staining

Dipicolinic Acid

• Dipicolinic acid is an important component of endospores.
• It binds with calcium ions as calcium-dipicolinate, enhancing resistance to heat, radiation, desiccation, and chemicals.
• It reduces water content in the spore core, minimizing metabolic activity and protecting DNA.
• Calcium-dipicolinate stabilizes spore enzymes and DNA, providing thermal resistance.
• Endospores are highly resistant, dormant structures formed by certain bacteria (Bacillus and Clostridium).