Microbial cell walls

Importance of cell surface

• where microbes and immune system interact

Capsules

• Slime layers / glycocalyx

• Loose network of polymer fibres extending out from wall

Glycocalyx

• involved in biofilm formation

Capsule

• organised tight matrix

• Excludes small particles

Slime layer

• unorganised

• Easily removes

• Doesn’t exclude small particles

Role of capsules

• nutrient capture

• Protection

• Attachment

Types of capsule

• Glycolipid

• Protein

• Extracellular slime

S-layer

• paracrystalline outer wall

• Protein and glycoprotein

• Can be protective

Peptidoglycan

• alternating NAM and NAG residues

• Arranged in dimers , cross linked by AA side chains

• Mesh like polymer, retains gram stain in gram + cells

NAM

• N acetyl muramic acid

NAG

• N acetyl glucosamine

PG synthesis

• chains of linked PG subunits joined by cross links

Gram positive cell wall

• thicker than gram -ve

• Lots of PG (90% of cell wall)

Gram -ve cell wall

• little PG

• Between inner and outer membrane

Lysozyme

• antibacterial enzymes

• Degrades PG backbone , making cells sensitive to changes in osmotic pressure

Teichoic acid

• in gram +ve cell walls only

• May provide -ve charge, helps with binding of +ve ions

Archaeal cell walls

• No PG

• Contains substitute for NAM Which isn’t degraded by lysozymes or sensitive to penicillin

Hopanoids

• In bacteria

Sterols

• in eukaryotes

Sterols / hopanoids

• rigid planar molecules

• Stabilise membrane structure

Gram -ve outer membrane

• asymmetric due to LPS insertion into outer layer

How is the outer membrane linked to cells

• adhesion sites

• Brauns lipoprotein

Brauns lipoprotein

• most abundant protein in outer membrane

• Covalently linked to PG

Adhesion sites

• continuum of inner and outer membrane

• The 2 membranes adhere

Archaeal membranes Lipids

• branched chain HC attached to glycerol

• Single headed lipids make lipid bilayer

• Or double headed ether lipids making a monolayer

Purpose of lipid monolayer

Stabilises membrane under extreme conditions