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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 gramnegative 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