Download Gram positive cell wall

Anatomy of Bacterial Cell
 Outer layer - consist of two components:
1. Rigid cell wall
2. Cytoplasmic (Cell/ Plasma) membrane
present beneath cell wall
 Cytoplasm - gel-like substance enclosed within the
cell membrane contains cytoplasmic inclusions,
ribosomes, mesosomes and nucleoid .
 Additional structures - plasmid, slime layer, capsule,
flagella, fimbriae (pill), spores, see fig. 1
CELL WALL
Outermost layer, encloses cytoplasmic membrane ,It surrounds
the cell membrane and provides these cells with structural
support and protection, and confers shape and rigidity.
Peptidoglycan serves a structural role in the bacterial cell wall,
giving structural strength, as well as counteracting the osmotic
pressure of the cytoplasm. peptidoglycan helps maintain the
structural strength of the cell, Peptidoglycan is also involved
in binary fission during bacterial cell reproduction.
Chemical nature of the cell wall helps to divide bacteria
into two broad groups — Gram positive & Gram negative.
Gram +ve bacteria have simpler chemical nature than
Gram —ve bacteria
The peptidoglycan layer is substantially thicker in Grampositive bacteria (20 to 80 nanometers) than in Gramnegative bacteria (7 to 8 nanometers), Peptidoglycan forms
around 90% of the dry weight of Gram-positive bacteria but
only 10% of Gram-negative strains. Thus, presence of high
levels of peptidoglycan is the primary determinant of the
characterisation of bacteria as Gram-positive. In Gram-positive
strains, it is important in attachment roles and serotyping
purposes.
Cell wall consist of Mucopeptide (peptidoglycan/ murein):
formed by N-acetyl glucosamine (NAG) & N-acetyl
muramic acid (NAM) alternating in chains, held by
peptide chains.( cross link with tetrapeptide and pentaglycan
bridge. see fig .2
Cell wall can not be seen by direct light microscopy and do not
stain with simple stains, also can be observed by plasmolysis
and carries bacterial antigens — important in virulence &
immunity.Several antibiotics may interfere with cell wall
synthesis e.g. Penicillin, Cephalosporins
Gram positive cell wall
• The Gram-positive cell wall is composed of a thick
multilayered peptidoglycan sheath outside of the cytoplasmic
membrane.
• Teichdic 'acids are linked to and embedded in the
peptidoglycan, and lipoteichoic acids extend into the
cytoplasmic membrane
Gram negative cell wall
the Gram-negative cell wall is composed of an outer
membrane linked to thin single-layered peptidoglycan by
lipoproteins.
the peptidoglycan is located within the periplasmic space
that is created between the outer and inner membranes.)
The outer membrane. includes (porins which allow the
passage of small hydrophilic
molecules across
the membrane and lipopolysaccharide molecules
that extend into extracellular space. See table 1.
cytoplasmic(plasma)membrane

The boundary of the cell, sometimes called the
plasma membrane, thin layer 5-10 nm separates internal
metabolic events from the external environment and
controls the movement of materials into and out of the cell.
This membrane is very selective about what it allows to
pass through; this characteristic is referred to as “selective
permeability.” For example, it allows oxygen and nutrients
to enter the cell while keeping toxins and waste products
out. The plasma membrane is a double phospholipid
membrane, or a lipid bilayer, with the nonpolar
hydrophobic tails pointing toward the inside of the
membrane and the polar hydrophilic heads forming the
inner and outer surfaces of the membrane with small
amounts of Carbohydrates. see fig .3,4
cytoplasm
Colloidal system of variety of organic and enorganic
solutes in viscous watery solution
Cytoplasm Contains:
ribosomes,
mesosomes,
inclusions
vacolus
Ribosomes:
In most bacteria the most numerous intracellular
structure is the ribosome, the site of protein synthesis in
all living organisms. All prokaryotes have 70S (where
S=Svedberg units) ribosomes while eukaryotes contain
larger
80S ribosomes in
their cytosol.
The
70S ribosome is made up of a 50S and 30S subunits. The
50S subunit contains the 23S and 5SrRNA while the 30S
subunit contains the 16S rRNA. These rRNA molecules
differ in size in eukaryotes and are complexed with a
large number of ribosomal proteins, the number and type
of which can vary slightly between organisms.
Mesosomes:
It is a multilaminated structures formed as invagination
of plasma membrane , it is principle sites of respiratory
enzymes, and coordinate nuclear and cytoplasmic
division during binary fission.
Intracytoplasmic inclusion :
Inclusions are considered to be nonliving components of
the cell that do not possess metabolic activity and are not
bounded by membranes. It is reserve of energy and
phosphate for cell metabolism. The most common
inclusions are glycogen, lipid droplets, crystals, and
pigments. Volutin granules are cytoplasmic inclusions of
complexed inorganic polyphosphate. These granules are
called metachromatic granules due to their displaying the
metachromatic effect; they appear red or blue when
stained with the blue dyes methylene blue or toluidine
blue in diphtheria bacilli.
Nucleus:
No nucleolus, no nuclear membrane, oval or elongated
bodies generally one per cell.
The genome is single circular double stranded DNA ,
haploid and divided by binary fission.
Additional organelles
1. Plasmid:
Extranuclear genetic elements consisting of DNA
And transmitted to daughter cells during binary
fission .
May be transferred from one bacterium to another
and not essential for life of the cell .the function
of plasmid is to confer certain properties such as
drug resistance ,toxicity.
2. .Capsule & Slime layer:
Viscous layer secreted around the cell wall
Polysaccharide / polypeptide in nature
A. Capsule sharply defined structure, antigenic in
nature • Protects bacteria from Iytic enzymes ,
inhibits phagocytosis • Stained by negative staining
using India Ink • Can be demonstrated by Quellung
reaction (capsule swelling reaction) .
B. Slime layer loose undemarcated secretion
Repeated subcultures in vitro lead to loss of capsule
and also of virulence .
3. Flagella
Flagella (singular: flagellum) are long, thin,
whip-like appendages long (3 to 12 micron )
attached to a bacterial cell that allow for bacterial
movement (also known as motility) ,filamentous
surface appendages this organs of locomotion is
present in motile bacteria.
The number and distribution of flagella on the
bacterial surface are characteristic for a given
species - hence are useful in identifying and
classifying bacteria so that different bacterial
species have different flagella arrangements, from a
single flagellum to one on each end to tufts of
many..),
Chemically, composed proteins called flagellins ,
Each flagellum consist of three part :filament, hook
and basal body .
Flagella may serve as antigenic determinants (e.g.
the H antigens of Gram-negative enteric bacteria (
Presence shown by motility e.g. hanging drop
preparation.
4 .Fimbriae/ Pili Thin, hairlike appendages
on the surface of many Gram negative bacteria
10 to 20 micron long, acts as organs of adhesion
(attachment) -allowing bacteria to colonize
environmental surfaces or cells and resist flushing .
It is made up of proteins called pilins. • Pili can be
of two types — Common pili - short& abundant
Bordetella pertussis is the bacteria that causes
whooping cough. Bordetella has pili coated with
adhesins that can surface of the respiratory tract and
will stick to only that surface, allowing it to adhere
identify the mucosal to and infect those cells.
Sex pil - small number (one to six), very long pili,
helps in conjugation (process of transfer of DNA)
Pili can also aid in attachment between bacterial
cells. Some bacteria are able to produce conjugation
pili that allow for the transfer of DNA from one
bacterial cell to another. Bacteria have evolved the
process of conjugation as a way to increase genetic
variability. The cell with the conjugation pilus
attaches to another cell, connecting the cytoplasm of
each cell and allowing molecules of DNA to pass
through the hollow pilus.
5. Spores :
Highly resistant resting stages formed during
adverse environment-(depletion of nutrients)
•Formed inside the parent cell, hence called
Endospores.
•Very resistant to heat, radiation and drying and can
remain dormant for hundreds of years .
•Formed by bacteria like Clostridium, Bacillus.
Endospores are bacterial survival structures that are
highly resistant to many different types of chemical
and environmental stresses and therefore enable the
survival of bacteria in environments that would be
lethal for these cells in their normal vegetative form.
It has been proposed that endospore formation has
allowed for the survival of some bacteria for
hundreds of millions of years (e.g. in salt
crystals) . Endospore formation is limited to several
genera of gram-positive bacteria such
as Bacillus and Clostridium. It differs from
reproductive spores in that only one spore is formed
per cell resulting in no net gain in cell number
upon endospore germination. The location of
an endospore within a cell is species-specific and
can be used to determine the identity of
a bacterium. See fig 4,5.