Bacterial Cell Wall

INTRODUCTION
 Cell wall was first observed and named simply as
a “wall” by Robert Hooke in 1665.
 In 1804, Karl Rudolphi and J.H.F. Link proved
that cells have independent cell walls.
 A cell wall is a structural layer that surrounds

some types of cells, situated outside the cell

membrane.
 It can be tough, flexible and rigid which provides
cell with both structural support and protection.
 CELL WALL
The cell wall is the outer most layer of the cell. In many
cases the cell wall comes in direct contact with the
environment.

Characteristics and functions

• Protection of the cell.

• Maintains the shapes of the cell.
• Maintains the osmotic integrity of the cell.
• Assist some cells in attaching to other cells or in eluding
antimicrobial drugs.
• Providing attachment sites for bacteriophages.
• Play an essential role in cell division.
• Providing a rigid platform for surface appendages-
flagella, fimbriae and pili.
• Regulation of substance transport into and out of cells.
• Contain supplemental genetic information such as
resistance to antibiotics, production of toxins and
tolerance to toxic environment.
• Mineral storage of cells

• Site of action of several antibiotics

• Some bacteria such as Mycoplasma lack cell wall
 Types of Bacteria based on cell wall differences
 Based on the structure of
the cell wall and to their
response to stain, bacteria
have been classified into
two types:
- Gram Positive Bacteria
- Gram Negative Bacteria

GRAM STAINING
• In the first step, the smear is stained with the
basic dye crystal violet, the primary stain.
• This is followed by treatment with an iodine
solution functioning as a mordant. The iodine
increases the interaction between the cell and
the dye so that the cell is stained more strongly.
• The smear is next decolorized by washing with
ethanol or acetone. This step generates the
differential aspect of the Gram stain;
grampositive bacteria retain the crystal violet,
whereas gram-negative bacteria lose their crystal
violet and become colorless.
• Finally, the smear is counterstained with a simple,
basic dye different in color from crystal violet.
Safranin, the most common counterstain, colors
gram-negative bacteria pink to red and leaves
gram-positive bacteria dark purple
Bacterial classification
Gm+ve cocci & Gm-ve
bacilli
 PEPTIDOGLYCAN

 Peptidoglycan ,also known as murein, is a polymer

consisting of sugars and amino acids that forms a mesh –
like layer outside the cell membrane of most bacteria
forming cell wall.
 The sugars component consist of alternating residues of ᵦ-
(1,4) linked N- acetylglucosamine and N- acetylmuramic
acid.
 These subunits which are related to glucose in their
structure are covalently joined to one another to form
glycan chains.
 PEPTIDOGLYCAN
 Attached to the N- acetylmuramic acid
is a peptide chain of three to five
amino acids. The peptide chain can be
cross- linked to the peptide chain of
another strand forming the
peptidoglycan.
 Peptidoglycan can be destroyed by
certain agents like the enzyme
lysozyme, a protein that cleaves the β-
1,4-glycosidic bonds between N-
acetylglucosamine and N-
acetylmuramic acid in peptidoglycan
thereby weakening the wall; water can
then enter the cell and cause lysis.
Lysozyme is found in animal secretions
including tears, saliva, and other body
fluids, and functions as a major line of
defense against bacterial infection.

Peptidoglycan structure
• In gram-negative bacteria, peptidoglycan
cross-linkage occurs by peptide bond
formation from the amino group of DAP
of one glycan chain to the carboxyl
group of the terminal D-alanine on the
adjacent glycan chain
• In gram-positive bacteria, cross-linkage
may occur through a short peptide
interbridge, the kinds and numbers of
amino acids in the interbridge varying
from species to species. For example, in
the gram-positive Staphylococcus
aureus, the interbridge peptide is
composed of five glycine residues, a
common interbridge amino acid
GRAM POSITIVE
CELL WALL

 Usually thick,
homogenous, composed
mainly of peptidoglycan.
 It accounts 50- 90% of
the dry weight of the cell
wall.
 Contain large amount of
teichoic acids.
Special components of Gram positive cell wall
 Teichoic acid
• The term “teichoic acids” includes all cell wall, cytoplasmic
membrane, and capsular polymers composed of glycerol
phosphate or ribitol phosphate.
• These polyalcohols are connected by phosphate esters and
typically contain sugars or D-alanine. Teichoic acids are
covalently bonded to muramic acid in the wall peptidoglycan.
• Because the phosphates are negatively charged, teichoic acids
are at least in part responsible for the overall negative
electrical charge of the cell surface.
• Teichoic acids also function to bind Ca21 and Mg21 for
eventual transport into the cell.
• Certain teichoic acids are covalently bound to membrane
lipids, and these are called lipoteichoic acids
 GRAM NEGATIVE CELL WALL
 Multi layered and more
complex than gram positive
cell walls.
 Peptidoglycan of gram negative
bacteria is thin comprises only
10% or less of cell wall.
 Outer membrane lies outside
the thin peptidoglycan layer.
Special components of Gram negative cell wall
Periplasm:
• The region between the cytoplasmic membrane and
the outer membrane is filled with a gel-like fluid
called periplasm.

• In gram negative bacteria, all secreted proteins are

contained within the periplasm, unless they are
specifically translocated across the outer membrane.

• Periplasm is filled with the proteins that are involved

in various cellular activities, including nutrient
degradation and transport.
Outer membrane
• Peptidoglycan layer is surrounded by outer
membrane in the gram negative bacteria.

• Its outside leaflet is made up of lipopolysaccharides,

rather than phospholipids.

• For this reason, the outer membrane is also called

the lipopolysaccharide layer or LPS.
• The outer membrane functions as a protective
barrier and excludes many toxic compounds.
• Lipopolysaccharide molecule is extremely important
from a medical stand point.
• It consists of three parts, two of them are medically
significant.

1. Lipid A…..embedded in membrane.

2. Core polysaccharide…..located on the surface of
membrane.
3. O antigens….which are short polysaccharides
extended out from core.
• Lipid A: The chemical makeup of lipid A molecule
plays significant role in our body’s ability to recognize
the presence of invading bacteria.
• It is toxic in nature, as a result the LPS can act as an
endotoxin, causing symptoms like fever, diarrhea
and shock.

• O-antigen: It is composed of carbohydrates,

including glucose, galactose, mannose and some
other sugars in varying combinations.
• The O-antigens can react with their specific
antibodies.
 Cells That Lack Cell Walls

• Mycoplasmas, a group of pathogenic bacteria that causes

several infectious diseases of humans and other animals,
and the Thermoplasma group, species of Archaea naturally
lack cell walls.
• These bacteria are able to survive without cell walls
because they either contain unusually tough cytoplasmic
membranes or because they live in osmotically protected
habitats such as the animal body.
• Most mycoplasmas have sterols in their cytoplasmic
membranes, and these probably function to add strength
and rigidity to the membrane as they do in the cytoplasmic
membranes of eukaryotic cells.
Archaeal cell wall
• The cell walls of certain methanogenic Archaea
contain a molecule that is remarkably similar to
peptidoglycan, a polysaccharide called
pseudomurein or pseudopeptidoglycan
• The backbone of pseudomurein is composed of
alternating repeats of N-acetylglucosamine (also
found in peptidoglycan) and N-
acetyltalosaminuronic acid; the latter replaces the
Nacetylmuramic acid of peptidoglycan.
• Pseudomurein also differs from peptidoglycan in
that the glycosidic bonds between the sugar
derivatives are β-1,3 instead of β-1,4.
S-layer
• The most common cell wall in species of
Archaea is the paracrystalline surface layer,
or S-layer.
• S-layers consist of interlocking protein or
glycoprotein molecules that show an
ordered appearance when viewed with the
electron microscope
• The paracrystalline structure of S-layers is
arranged to yield various symmetries, such
as hexagonal, tetragonal, or trimeric,
depending upon the number and structure
of the protein or glycoprotein subunits of
which they are composed.
• The cell walls of some Archaea, for example the methanogen
Methanocaldococcus jannaschii, consist only of an S-layer. Thus, S-layers are
themselves sufficiently strong to withstand osmotic bursting.
• However, in many organisms S-layers are present in addition to other cell wall
components, usually polysaccharides. For example, in Bacillus brevis, a
species of Bacteria, an S-layer is present along with peptidoglycan.
• However, when an S-layer is present along with other wall components, the S-
layer is always the outermost wall layer, the layer that is in direct contact
with the environment.
• Besides serving as protection from osmotic lysis, S-layers may have other
functions. For example, as the interface between the cell and its environment,
it is likely that the S-layer functions as a selective sieve, allowing the passage
of low-molecular-weight solutes while excluding large molecules and
structures (such as viruses).
• The S-layer may also function to retain proteins near the cell surface,
much as the outer membrane does in gram-negative bacteria. We thus
see several cell wall chemistries in species of Archaea, varying from
molecules that closely resemble peptidoglycan to those that totally lack a
polysaccharide component. But with rare exception, all Archaea contain a
cell wall of some sort, and as in Bacteria, the archaeal cell wall functions
to prevent osmotic lysis and gives the cell its shape.
• In addition, because they lack peptidoglycan in their cell walls, Archaea
are naturally resistant to the activity of lysozyme and the antibiotic
penicillin, agents that either destroy peptidoglycan or prevent its proper
synthesis