Faculty: Dr. Alvin Fox

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CELL ENVELOPE, SPORES AND MACROMOLECULAR
BIOSYNTHESIS
Faculty Dr. Alvin Fox
Suggested reading Murray, Third
edition Chapter 3
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KEY WORDS
Cell envelope Lipopolysaccharide
(endotoxin) Cell wall Teichoic acid Cell
membrane Teichuronic acid Outer membrane
Lipoteichoic acid Peptidoglycan
Mycolic acid Braun lipoprotein Undecaprenol
(bactoprenol) Porins
Endospore
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Topics for discussion

1. The structure of the Gram negative, Gram positive
   and acid fast cell envelopes.
2. The composition and function of unique cell
   envelope macromolecules and their biosynthesis.
3. Endospores, which are unusual in many ways.

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Cell Envelope
The cell envelope may be defined as the cell
membrane and cell wall plus an outer membrane if
one is present. The cell wall consists of the
peptidoglycan layer and attached structures.
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Bacterial cell envelopes fall into two major
categories
Gram positive and Gram negative
Gram staining is based on characteristics that
reflect major structural differences between the
two groups.
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Peptidoglycan
A single bag-shaped, highly cross-linked
macromolecule that surrounds the bacterial cell
membrane and provides rigidity.
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Peptidoglycan consists of

• 1. Glycan (polysaccharide) backbone consisting
  of
• N-acetyl muramic acid (Mur)
• N-acetyl glucosamine (Gln)
• 2. Peptide side chains containing
• A. D- and L- amino acids
• B. Diaminopimelic acid (in some cases)
• C. The side chains are cross-linked by peptide
  bridges which
  vary in structure among bacterial species.

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Muramic acid, D-amino acids and diaminopimelic
acid are not synthesized by mammals. PG is found
in all eubacteria except Chlamydia and Mycoplasma.
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Gram Positive Cell Envelope
Lipoteichoic acid
Peptidoglycan-teichoic acid
Cytoplasmic membrane
Cytoplasm
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Gram Positive Cell Envelope

• Covalently bound to the thick peptidoglycan are
• Teichoic acid their backbones are usually
  phosphorus-containing polymers of ribitol or
  glycerol or
• Teichuronic acid which are glucuronic acid-
  containing polysaccharides.

These negatively charged molecules are believed
to be involved in concentrating metal ions from
the surroundings. Teichoic acids can also
direct autolytic enzymes to sites of
peptidoglycan digestion (autolysis), one of the
steps in cell wall biosynthesis.
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Gram Negative Cell Envelope
Lipopolysaccharide
Porin
Outer Membrane
Braun lipoprotein
Peptidoglycan
Cytoplasm
Inner (cytoplasmic) membrane
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Gram Negative Cell Envelope

• Covalently linked to the thin peptidoglycan is
• Braun lipoprotein which binds the outer membrane
  to the cell wall.
• Proteins and phospholipids
• Lipopolysaccharide which consists of three
  regions
• an outer O antigen,
• a middle core which contains several sugars
  (heptoses and ketodeoxyoctonic acid), not found
  elsewhere in nature
• an inner lipid A region which contains ß hydroxy
  fatty acids (uncommon in nature). The molecule
  displays endotoxin activity.

Porins in the outer membrane help form channels
to allow passage of small hydrophilic nutrients
(such as sugars) through the outer membrane.
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Structure of Lipopolysaccharide
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Acid fast and related bacteria (mycobacteria,
nocardia and corynebacteria)
The cell envelopes of these organisms are
considerably more complex than other bacteria.
Mycolic acid (long, branch chained fatty acids)
is covalently bound via a polysaccharide to
peptidoglycan. Additional mycolic
acid-containing compounds and other complex
lipids form a thick waxy membranous layer
outside the peptidoglycan layer.
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Synthesis of cell envelope macromolecules
Peptidoglycan 1. The precursor subunit (muramyl
pentapeptide attached to uridine diphosphate,
UDP) is synthesized in the cytoplasm and passed
to the cell membrane.
2. The subunit is moved enzymatically from the
nucleotide to a lipid carrier (undecaprenol/bacto
prenol) and built into a completed subunit
(disaccharide pentapeptide with attached bridge
peptide).
3. The completed subunits are then exported to
the cell wall.
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4. The undecaprenol is recirculated in the cell
membrane and used again.
5. The glycan backbones of the existing cell
wall is enzymatically broken (by autolysins) to
allow insertion of the newly synthesized subunit.
6. Cross-linking of the peptide side-chain of the
inserted subunit to the existing chain then
occurs enzymatically (penicillin binding
proteins).
7. Completed subunits of teichoic and teichuronic
acids are also synthesized in the cell membrane
(on lipid carriers) before transport and
insertion into the existing cell wall.
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Lipopolysaccharide
1. Lipid A is assembled in the cell membrane and
the core sugars attached sequentially.
2. O-antigen subunits are independently
synthesized (on a lipid carrier as in
peptidoglycan synthesis).
3. The fully synthesized O-antigen is then
attached to the lipid A-core (generating
lipopolysaccharide) in the cell membrane before
passage/insertion into the outer membrane.
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Endospores
1. These modified Gram positive bacterial cells
have an unusual cell envelope that contains a
cell membrane and an outer membrane.
2. The peptidoglycan layer is less cross-linked
than in most bacterial cells and contains a
dehydrated form of muramic acid.
3. The spore peptidoglycan is referred to as a
cortex and is found between the two membranes.
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4. A coat consisting of highly cross-linked
keratin is found around the outside of the cell.
5. The bacterial spore is highly resistant to
chemical agents because of this coat.
6. When sporulation occurs, cell division is
unequal and the larger so-called "mother cell"
envelops the daughter cell.
7. The cell membrane of the daughter cell
constitutes the inner membrane of the spore and
the cell membrane of the mother forms the outer
membrane.
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