Cell walls

Gram-positive cell walls

Thick layer of peptidoglycan surrounding the plasma membrane

Contain teichoic acids

Teichoic acids

Polymers of glycerol or ribitol joined by phosphate groups

Amino acids or sugars are attached to glycerol or ribitol groups

Teichoic acids

Can be attached to either peptidoglycan or membrane lipid lipoteichoic acid

May contribute to negative charge of cell surface

Are not found in gram-negative bacteria

Gram-positive periplasmic space

Periplasmic space separates membrane from peptidoglycan layer

Gram-negative cell walls

Thin layer of peptidoglycan surrounded by outer membrane

Outer membrane provides addition barrier

Porins

Channel-forming proteins found in the outer membrane

Allow for selective permeability of outer membrane

Porins

Usually associate in copies of 3 individual monomers

Can allow nonspecific or specific passage of molecules < 700 daltons across the membrane

Lipopolysaccharide (LPS)

Large complex molecule

Found in the outer leaflet of the outer membrane

Not found in gram-positive bacteria

Lipopolysaccharide (LPS)

Consists of three parts:

1. Lipid A

2. Core polysaccharide

3. O side chain

Lipid A

Composed of 2 glucosamines attached to fatty acids and phosphates

Is embedded in the outer membrane

Is the toxic part of LPS (endotoxin)

Core polysaccharide

Is joined to Lipid A

In Salmonella is composed of 10 sugars (many unusual)

e.g. KDO (2-keto-3-deoxyoctonate)

O-side chain

Most variable region of LPS

Composed of a variable number of sugars

Rapid changing of LPS can help pathogens avoid immune system

Contributes to the negative charge of cell surface

O-side chain

Stability of LPS is enhanced by presence of divalent cations (Mg 2+)

Cations form bridges between O-side chains on different LPS molecules

Braun’s lipoprotein

Most abundant outer membrane protein

Covalently attached to peptidoglycan

Hydrophobic end embedded in outer membrane

Gram-negative periplasmic space

Two theories regarding periplasmic space

Thin layer of peptidoglycan surrounded by fluid-filled space

Gram-negative periplasmic space

Two theories regarding periplasmic space

Thin layer of peptidoglycan surrounded by fluid-filled space

or

Peptidoglycan exists as a gel-like material that fills the space

Peptidoglycan

Composed of alternating NAG and NAM with short peptide chain attached to the NAM

Some amino acids in the D form (not found in proteins)

Protects against attack by peptidases

Peptidoglycan

Peptidoglycan synthesis

Basic unit is NAG-NAM-pentapeptide

Synthesized in cytoplasm

Binding of UDP to sugars activates NAG and NAM

Bactoprenol acts as a carrier

Bactoprenol

Very hydrophobic molecule

Allows for transport through the interior of the membrane

Peptidoglycan synthesis

Bactoprenol transports NAG-NAM-pentapeptide across the membrane

NAG-NAM-pentapeptide is transferred to growing peptidoglycan

Bactoprenol is transferred back to the cytoplasm

Effects of antibiotics on peptidoglycan synthesis

Cycloserine blocks conversion of L-Ala to D-Ala

Vancomycin blocks peptidoglycan synthesis by inhibiting formation of cross-bridges between strands

Effects of antibiotics on peptidoglycan synthesis

Bacitracin blocks the dephosphorylation of bactoprenol-PP

Peptide cross-bridges

Gram-negative:

Amino acids directly joined via cross-bridge

Peptide cross-bridges

Gram-negative:

Amino acids directly joined via cross-bridge

Gram-positive:

Glycine pentapeptide bridge joins amino acids

Effect of penicillin on peptidoglycan synthesis

Antibiotics belonging to the penicillin family interfere with peptidoglycan synthesis

Inhibit formation of peptide cross-bridges

Effect of penicillin on peptidoglycan synthesis

Bind to proteins involved in peptidoglycan assembly

Penicillin binding proteins (PBPs)

Effect of lysozyme on peptidoglycan

Lysozyme is found in many bodily fluids

Cleaves bonds between alternating NAG and NAM subunits