Bacterial Cell Structure
Peptidoglycan:
- A mesh-like polymer of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) sugars, cross-linked by short peptide chains.
- Provides rigidity, maintains cell shape, and resists osmotic lysis against the cell's high internal osmotic pressure.
- The target of beta-lactam antibiotics (inhibit cross-linking transpeptidases) and glycopeptides (block the sugar-peptide precursor itself). Present in essentially all bacteria except Mycoplasma (no cell wall at all) and cell-wall-deficient forms.
Gram positive cell wall:
- Thick peptidoglycan layer (multiple cross-linked sheets), retaining the crystal violet-iodine complex during Gram staining.
- No outer membrane.
Gram negative cell wall:
- Thin peptidoglycan layer (single or few sheets), located within the periplasmic space, does not retain crystal violet, counterstains pink/red with safranin.
- Outer membrane present external to the peptidoglycan.
Teichoic acids:
- Polymers of glycerol or ribitol phosphate, covalently linked to peptidoglycan, found only in Gram positive cell walls.
- Contribute to cell wall rigidity, regulate autolysin activity, and act as a virulence factor (adhesion, immune activation).
Lipoteichoic acids:
- Similar to teichoic acids but anchored to the cell membrane rather than peptidoglycan, extending through the wall to the cell surface.
- Recognised by host pattern recognition receptors (Toll-like receptor 2), triggering inflammatory cytokine release in Gram positive sepsis, functionally analogous to LPS in Gram negative sepsis.
Outer membrane (Gram negative only):
- A second lipid bilayer external to the peptidoglycan, asymmetric: phospholipids on the inner leaflet, lipopolysaccharide (LPS) on the outer leaflet.
- Acts as a selective permeability barrier, excluding many antibiotics and host defence molecules, a key basis of intrinsic Gram negative antibiotic resistance.
Lipopolysaccharide (LPS):
- Composed of Lipid A (embedded in the outer membrane, the toxic component, also called endotoxin), a core polysaccharide, and the O antigen (outermost, highly variable, used for serotyping).
- Lipid A is recognised by TLR4, triggering the cytokine cascade responsible for fever, hypotension, and septic shock in Gram negative infection (full detail under Microbial Toxins).
Porins:
- Transmembrane protein channels through the outer membrane, allowing passive diffusion of small hydrophilic molecules (nutrients, some antibiotics) across an otherwise impermeable barrier.
- Loss or mutation of porins is a recognised mechanism of acquired antibiotic resistance (reduced drug entry).
Periplasmic space:
- The space between the inner (cytoplasmic) membrane and the outer membrane in Gram negative bacteria, containing the thin peptidoglycan layer plus a high concentration of hydrolytic enzymes, including beta-lactamases.
- This is the site where many Gram negative organisms inactivate beta-lactam antibiotics before the drug can reach its target.
