Microbiology ch 4

Peptidoglycan and Bacterial Cell Wall Structure

  • Peptidoglycan Composition:

    • Peptidoglycan is a complex network composed of repeating disaccharide chains cross-linked by short peptides.

    • The backbone consists of two alternating modified glucose derivatives:

    • NN-acetylglucosamine (NAG / NAD), which contains an added acetyl amino group.

    • NN-acetylmuramic acid (NAM), which contains an acetyl group chain plus an additional side-chain extension.

  • Glycosidic Bond Linkages:

    • Individual NAM and NAG sugars are linked together by β-1,4-glycosidic bonds\beta\text{-1,4-glycosidic bonds} involving specific carbon atoms in the sugar rings.

    • Humans and higher animals lack enzymes capable of hydrolyzing β-1,4-glycosidic bonds\beta\text{-1,4-glycosidic bonds} in peptidoglycan, making them unable to digest bacterial cell walls upon ingestion.

  • Peptide Cross-Linking:

    • Attached to NAM are amino acid side chains that act like "Velcro extensions."

    • These amino acid extensions cross-link adjacent polysaccharide strands to form a multi-layered, three-dimensional meshwork.

    • This dense structural network provides shape, physical strength, and osmotic protection to the bacterial cell.

Gram-Positive vs. Gram-Negative Cell Envelope

  • Gram-Positive Cell Envelope:

    • Contains a thick, multi-layered peptidoglycan cell wall positioned outside the cytoplasmic membrane.

    • Teichoic Acid and Lipoteichoic Acid:

    • Specialized acidic polymers embedded within the thick peptidoglycan layer.

    • Function to maintain cell wall integrity and assist in cell wall reassembly during cell division, ensuring both daughter cells receive fully intact walls.

    • Do not directly impart mechanical rigidity, but maintain structural wall function.

    • Because they are acidic, teichoic and lipoteichoic acids carry a net negative charge, imparting an overall negative charge to the Gram-positive cell surface.

  • Gram-Negative Cell Envelope:

    • Contains a thin, single-layer peptidoglycan wall surrounded by an outer membrane.

    • Lacks teichoic acid and lipoteichoic acid completely.

    • Outer Membrane Structure:

    • Asymmetric lipid bilayer containing lipopolysaccharides (LPS) located specifically on the outer leaflet.

    • Outer Membrane Porins (OMPs):

    • Non-specific, transmembrane channel proteins embedded in the outer membrane.

    • Facilitate the passive diffusion of nutrients and environmental molecules across the outer membrane.

    • Periplasmic Space:

    • A distinct compartment located between the inner plasma membrane and the outer membrane, containing the thin peptidoglycan layer and periplasmic enzymes.

    • Permeability Barriers:

    • Gram-negative bacteria are less penetrable to external substances than Gram-positives because molecules must cross two separate membrane barriers (outer membrane and plasma membrane) to reach the cytoplasm.

Atypical Cell Walls and Wall-Less Bacteria

  • Acid-Fast Cell Walls (Mycobacterium and Nocardia):

    • Contain unique cellular envelopes rich in mycolic acids and specialized lipids.

    • Do not stain effectively with standard Gram staining procedures.

    • Require specialized acid-fast staining procedures for identification.

  • Wall-Less Bacteria (Mycoplasma):

    • Lacks a peptidoglycan cell wall entirely.

    • Represents the smallest known bacterial cells with the smallest genomic size (a single minimal circular chromosome encoding essential genes).

    • Membrane Stabilization:

    • Cell membrane is stabilized by sterols (cholesterol).

    • Evolutionary origin: Host cholesterol was originally acquired from infected hosts during evolutionary history and incorporated into the bacterial membrane. It is passed to offspring and replenished from host tissues during infection.

    • Morphology:

    • Highly pleomorphic (lacking a defined or constant shape) due to the absence of a rigid cell wall.

Internal Bacterial Structures and Cytoplasm

  • Cytoplasm and Cytosol:

    • Cytoplasm: Refers to all internal cellular components contained within the plasma membrane, including water, dissolved sugars, proteins, ribosomes, genomic DNA, and inclusion bodies.

    • Cytosol: Refers strictly to the fluid portion of the cytoplasm.

  • Plasma Membrane Function and Sensing:

    • Houses transport proteins for nutrient uptake and waste secretion, as well as anchoring proteins.

    • Tox