Structure/Function of Bacteria: Cell Wall

Outline:
❑ Differences between Gram (-) and Gram (+) cell walls
❑ Structure of peptidoglycan
❑ The outer membrane of Gram (-) cells
❑ Cell walls of Archaea
 MMG 301 Structure/Function of Bacteria: Cell Wall
For most prokaryotic cells, the cytoplasmic membrane is surrounded
by a wall.
Integrity of the cell is dependent on
structures exterior to the cytoplasmic
membrane.
• High concentration of solutes
inside the cell creates internal
pressure due to osmosis.

• A semi-rigid cell wall protects the

cell from bursting.

Example of microbes with no cell wall

• Mycoplasmas – a group of bacteria that
have no cell wall
• Pleomorphic: have no specific
morphology (shape)
Most bacteria can be placed into one of two major groups based on the
structure of their cell wall.
• Differences in cell wall account for differences in the Gram stain result
• Cell walls contain peptidoglycan (also called murein)
• Many antibiotics inhibit enzymes that synthesize peptidoglycan
• Gram-positive cells: a single, thick peptidoglycan and a cytoplasmic
membrane
• Gram-negative cells: Two membranes – cytoplasmic and an outer
membrane. Peptidoglycan is between membranes.
Peptidoglycan of E. coli is a crosslinked polymer of a glycan tetrapeptide
repeating unit.

Peptide bonds
linked by
Amino acids
connected by A flexible mesh
to form called Murein
N-acetylglucosamine completely
sacculus
surrounding cell the
made and
Peptidoglycan Polymers of
of
N-acetylmuramic acid that
to make
longer
Insertion of new followed Cutting holes by
Can grow
monomers by
by with

• Transpeptidases Autolysin
• Transglycosylases enzymes
Peptidoglycan of E. coli is a polymer of a glycan tetrapeptide repeating
unit.

• Strands of repeated disaccharide N-

acetylglucosamine – N-
acetylmuramic acid connected by a
glycosidic bond.

• -M-G-M-G- polymer strands are

connected by amino acid crosslinks
(peptide crosslink).

• “Murein sacculus”: the name given to

the large peptidoglycan molecule that
completely surrounds the cell.
Peptidoglycan of different bacteria have a typical (G-M)n polymer but
the peptide crosslinks are often different.

• The peptide crosslink of

Staphylococcus aureus is different,
but the N-acetylglucosamine – N-
acetylmuramic acid polymer is
identical to that of E. coli.

Gram-positive
peptidoglycan has 10-15
sheets connected by
peptide crosslinks.
There are differences between the peptidoglycan of Gram (-)
and Gram (+) cells.
Gram (-) Gram (+)
Peptidoglycan 1-3 10-15 or more
sheets

Peptidoglycan few many

crosslinks
Other unique Lipoprotein Teichoic acids
features Periplasm Lipoteichoic
acids

Peptidoglycan differences explain the

result of the Gram stain.
• Gram (+) cell thick peptidoglycan
retains the crystal violet-iodine complex

• This crystal violet-iodine complex is

washed away from the thin
peptidoglycan of Gram (-) cells.
 Peptidoglycan of Gram (+) cells is reinforced by teichoic acids and
lipoteichoic acids.
• Teichoic acids
• Polymers of repeating ribitol or glycerol
phosphate.
• Connect peptidoglycan sheets together.
• Produce a negative charge on cell surface
that can bind positively charged nutrients.

• Lipoteichoic acids: teichoic acids that

terminate in fatty acids, which insert into
the cytoplasmic membrane.

Link: Short animation peptidoglycan

in Gram(-) and Gram (+) cell walls
https://youtu.be/QEc2aUaD25w
The Gram (-) cell wall contains a thin peptidoglycan layer surrounded
by an outer membrane.
The outer membrane of Gram-negative contains phospholipids, but other
components also.
• The outer membrane bilayers are asymmetric.
• Lipopolysaccharide, or LPS that extended outward from the cell.
• Lipoproteins that connect the outer membrane with peptidoglycan.

• Transport proteins (porins) span through the outer membrane.

• Periplasm: space between the cytoplasmic membrane and the outer membrane.

Periplasm
contains
enzymes for
nutrient transport
and cell growth.
Lipopolysaccharide (LPS) of the Gram (-) cell is a major component of
the outer membrane.
• LPS replaces much of the phospholipid in the outer face of the outer membrane.
• Major parts: Lipid A, Core polysaccharide, and O-specific polysaccharide.
• This general scheme is typical for most Gram (-) bacteria, but the exact
composition differs for each type of bacteria.

The lipid A portion, with parts of the core and

O-specific polysaccharide attached can act as
an endotoxin.
• Animal immune systems recognize endotoxin
and produces a response
Lipopolysaccharide (LPS) of the Gram (-) cell contains 3 major parts.

Lipid A: a disaccharide phosphate that is attached to fatty acids

Core polysaccharide: connected to the lipid A and to O-specific polysaccharide

O-specific polysaccharide: contains a repeated 4 or 5 saccharide sequences.

• Is also a major antigen to determine the serotype of some bacterial
pathogens.
• Antigen: any molecule that is recognized by the immune system.
• Serotype: identification of bacteria or viruses using antibody test.
Lipoprotein anchors the outer membrane to the peptidoglycan.

Lipoprotein is one of the most

abundant proteins in the cell.
• Helps in forming the periplasm.
• One end is linked to fatty acids
• The other end is chemically linked
to peptidoglycan.

Porins are trimeric tube-like

proteins that span the outer
membrane bilayer.
• Allow passive entry of some solutes
into the periplasm
• Some porins are specific for certain
molecules.
The cell walls of Archaea have different components.

Types of cell walls in Archaea include:

• Pseudomurein: similar in structure to

peptidoglycan; found only in some Archaea that
produce methane (methanogens).

• S-layers: a single layer of protein that

completely surrounds and protects the cell.
• Can be present with, or without other cell
wall components.
• Also found in a few bacteria.

Many other cell wall structures can be found in

prokaryotes. They represent the ability of
microorganisms to evolve and adapt to the wide
diversity of habitats found on earth.
Arrangements of S-layer proteins.
 Summary
• The cytoplasmic membrane of almost all prokaryotic cells is surrounded by a cell
wall
• Gram-negative and Gram-positive bacterial cell walls have different structures
• Gram-negative cell walls have a second, outer membrane that surrounds a thin
layer of peptidoglycan
• Gram-positive cell walls consist only of a thicker peptidoglycan layer
• Peptidoglycan is a structure unique to bacteria and consists of polymers of
alternating amino sugars connected by peptide crosslinks. The exact composition of
the amino acids of the crosslink will vary depending on the bacterium.
• The outer membrane of Gram-negative cells contains many unique structures,
including lipopolysaccharide (LPS), lipoprotein, and porins.
• Lipopolysaccharides are polymers of sugars that have fatty acids on one end of
the molecule – these fatty acids are embedded in the outer membrane. Animal
immune systems are pre-programmed to recognize parts of the LPS.
• Porins are protein “tubes” that transit the outer membrane and function in
nutrient transport.
• Lipoproteins have a connecting function - one end embedded in the inner face
of the cytoplasmic membrane and the other end is connected to peptidoglycan.
• S-layers is a single layer of closely packed protein that completely surrounds and
protects some types of prokaryotic cells.