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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-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
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 synthesis
Basic unit is NAG-NAM-pentapeptide
Synthesized in cytoplasm
Binding of UDP to sugars activates NAG and NAM
Bactoprenol acts as a carrier
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
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)