Structure of

bacteria
Dr. Mayram Hacıoğlu
mayram.tuysuz@istanbul.edu.tr

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• Cell wall Cytoplasm
• Cell membrane • Ribosomes
• Cytoplasm • DNA- nucleoid
• DNA- nucleoid • Plasmids
• Ribosome • Inclusion bodies

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Cell membrane (CM)
• The cell membrane (plasma membrane or cytoplasmic
membrane-CM) is a biological membrane that separates
the interior of a cell from its outside environment.
• The principal components of the cell membrane are lipids
(phospholipids), proteins and carbohydrates.
• A major difference in chemical composition between
prokaryotic and eukaryotic cells is that eukaryotes have
sterols in their membranes (e.g. cholesterol, ergosterol)
whereas prokaryotes do not.
• Sterol-like molecules, hopanoids

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• CM is selectively permeable
to ions and organic
molecules
• CM is not static structure
• Lipids move freely within the
two layers of the membrane
• The fluidity of the membrane
depends on the types of
lipids present, environmental
factors and the other
molecules associated with
the membrane

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The cytoplasmic membrane serves many functions,

• Transport of nutrients,
• Release of extracellular enzymes
• Respiration
• Cell wall synthesis
• DNA synthesis

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1. Transport of nutrients

• Mediates cellular processes by regulating the materials

that enter and exit the cell.
• Carries markers that allow cells to recognize one
another and can transmit signals to other cells via
receptors.

• Microorganisms can only take

in dissolved particles across a
selectively permeable
membrane
• Microorganisms use transport
mechanisms passive diffusion,
active transport
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2- Release of extracellular enzymes

• Secretes enzymes that break down large nutrient molecules

in the environment (eg hydrolysis enzyme)
• Secretes enzymes that act as virulence factors and enzymes
that neutralize certain substances that are toxic to the cell
(eg beta-lactamase)

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3- Respiration

• The cytoplasm membrane carries respiratory enzymes

(eg cytochrome enzyme) and cell respiration occurs in
this membrane.
• The energy released by breathing is used by the cell.

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 4- Cell wall synthesis

• Molecules required for cell wall construction and repair

are synthesized in the cytoplasm and removed from the
cell by special proteins in the cytoplasm membrane

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5- DNA synthesis
• CM, folds into the
cytoplasm to form sacs-like
formations.
• The septal (central)
mesosome has proteins
that are necessary for the
synthesis of the septum
that provides DNA Mesosome:
replication and cell an invagination of the CM, has
division. functions either in DNA
replication and cell division or
excretion of exoenzymes

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• It is located in the
middle of the cell.
• The bacterium DNA is
in contact with the
septal mesosome and
the cell division begins
from the site of the
septal mesosome.
• Therefore, cytoplasmic
membrane plays an
important role in DNA
synthesis, indirectly

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Cell Wall (CW)
• All prokaryotic cells has CW, except Mycoplasma and
Ureaplasma
• Protects the bacteria against its internal pressure and
gives them the specific shape.
• It is also chemically unlike any structure present in
eukaryotic cells and is therefore an obvious target for
antibiotics that can attack and kill bacteria without harm
to the host

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• CW, have a unique structural component called
peptidoglycan (also called murein or glycopeptide)

Peptidoglycan:
• Provides the hardness of bacterial cell wall
• Gives bacteria their characteristic shapes
• Made up of sugars and amino acids

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Structure of the peptidoglycan
• Large macromolecule containing
glycan (polysaccharide) chains
that are cross-linked by short
peptide bridges.
• The glycan chain acts as a
backbone to peptidoglycan
• The sugar component consists of
alternating residues of β-(1,4)
linked N-acetylglucosamine (NAG)
and N-acetylmuramic acid (NAM).

Lysozyme breaks the bond between NAG and NAM

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To each molecule of NAM
is attached a tetrapeptide
consisting of the amino
acids:

• L-alanine,
• D-alanine,
• D-glutamic acid
• Lysine or
Diaminopimelic acid.

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Types of Cell Wall

On the basis of cell wall

composition, bacteria are
classified into two major
group.

* Gram positive bacteria

* Gram negative bacteria

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1. Gram positive cell wall

Cell wall composition of Gram positive bacteria,

a. Peptidoglycan (50-90% of the cell wall is peptidoglycan)

b. Lipoteichoic acid
c. Teichoic acid

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a. Peptidoglycan
• Peptidoglycan is composed of three components.
• Glycan backbone
• Tetra-peptide side chain
( chain of 4 amino acids)
linked to NAM
• Peptide cross linkage

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 b. Teichoic acid
• Teichoic acid is water soluble polymer of glycerol
or ribitol phosphate.
• Appear to extend to the surface of the peptidoglycan
layer.
• Present in Gram positive bacteria.
• Constitutes about 50% of dry weight of cell wall.
• The major surface antigen of Gram positive bacteria.

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• They can be covalently linked to NAM or a terminal
D-alanine in the tetrapeptide crosslinkage between
NAM units of the peptidoglycan layer, or they can
be anchored in the cytoplasmic membrane with a
lipid anchor.

• Teichoic acids that are anchored to the lipid

membrane are referred to as lipoteichoic acids

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The functions of Teichoic acid are;
provide rigidity to the cell-wall
major antigenic determinant
transport ions
anchoring
external permeability barrier

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2. Gram Negative Cell Wall

Cell wall composition of Gram negative bacteria,

• Lipopolysaccharide (LPS)
* polysaccharide
* lipid A
• Outer membrane
* Lipid
* Protein
• Peptidoglycan (5-10% of the cell wall is peptidoglycan)

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a. Peptidoglycan
• The layer of peptidoglycan is 5-10% of the cell wall

b. Outer membrane
• Contains double-layer phospholipids.
• This membrane protects the bacteria against
antipeptidoglycan - specific substances (eg lysozyme).

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c. Lipopolysaccharide (LPS)
• LPS is attached to outer membrane by hydrophobic bond
• Synthesized in cytoplasmic membrane and transported to
outer membrane.
• LPS is an endotoxin produced by Gram negative bacteria
• Endotoxin is lethal in high concentrations
• Composed of lipid-A and polysaccharide

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Lipid-A
Antigenic
It is the endotoxin of the Gram negative bacterial cell.
The bacterial cell breaks down in the human body and is
toxic to mammalian cells when lipid A is released.
Pyrogen
Fever, diarrhea, leakage in blood vessels - tumor necrosis,
drop in blood pressure and shock

Polysaccharides
Mainly surface antigens of Gram negative bacteria.
This antigen is called the O antigen (body antigen).

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Importance of LPS
• contributes to negative charge
on cell surface
• helps stabilize outer
membrane structure
• may contribute to attachment
to surfaces and
biofilm formation
• creates a permeability barrier
• protection from host defenses
(O antigen)
• can act as an endotoxin (lipid
A)

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Cell wall composition of Gram negative bacteria,

• Lipopolysaccharide (LPS)
* polysaccharide
* lipid A- endotoxin
• Outer membrane
* Lipid
* Protein
• Peptidoglycan (5-10% of the cell wall is peptidoglycan)

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 Gram Staining
• Gram staining is a method
of staining bacteria,
• Bacteria are divided into
two groups according to
Gram staining method
• Gram-positive
• Gram-negative

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The technique was developed in the 19th Century by
Hans Christian Gram.
Gram developed the method while searching for a
way to make sectioned cells more visible, but it was
later discovered that bacteria could be classified into
two groups based on whether or not they keep the
stain’s color.

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Peptidoglycan cell wall differences are responsible for
whether a bacterium is Gram-positive or Gram-negative.

The Gram staining method basically involves

• staining the slides with crystal violet dye,
• adding iodine.
• wash with alcohol
• staining with safranin

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Gram staining method:
Prepare your samples on slide and fix with heat!!!
a. Flood slide with crystal violet. Allow to stand for
two minutes
b. Wash off with Gram’s iodine (a mordant)
c. Flood with Gram’s iodine (a mordant). Leave for
two minutes.
d. Wash off with tap water.

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e. Decolorize with alcohol (95%) until no more color washes
off (usually 10–20 seconds). This is a most critical step.
f. Wash off with water.
Do water-alcohol-water washing 2 or 3 times!!!
g. Apply safranin or fuchsin (the counterstain) for 30 seconds.
h. Wash off with water.
i. Dry the slide between the papers.
j. Examine all slides under oil with the oil-immersion
objective (x100).

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• Fixation: The stained cells in a In light microscopy, oil
microscope should resemble immersion is a technique
living cells as closely as used to increase
possible. Fixation is process the resolving power of
by which the internal and a microscope
external structures of cells
and microorganisms are
preserved and fixed. It
inactivates enzymes that
might disrupt cell morphology
so that they don't change
during staining and
observation. A microorganism
is usually killed and firmly
attached to microscopic slide
during fixation.
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• Gram-positive bacteria have thick peptidoglycan
layer which take in the crystal violet-iodine complex.
This makes the cell walls of these bacteria appear
purple; the purple masks the color of the lighter red
safranin counter-stain.
• Gram-negative bacteria, however, have thin cell
peptidoglycan that the crystal violet-iodine complex cannot
adhere to. In these bacteria, the purple stain washes off
but the red counterstain remains, making Gram-negative
bacteria appear red under Gram staining

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 Types of cell wall
1. Gram positive cell wall 2. Gram negative cell wall
• Peptidoglycan • Lipopolysaccharide
(LPS)
• Lipid
* polysaccharide
• Teichoic acid
* lipid A
• Outer membrane
* Lipid
* Protein
• Peptidoglycan

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Although most bacteria have a cell wall, some bacteria
cells do not have a cell wall.

e.g. Mycoplasma species

• Pleomorphic
• Intracellular parasites
• Must live within the cell
• They die immediately in very or low salt environments,
• The presence of sterols in cell membranes provides
resistance.

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 The loss of cell walls of bacteria
• Some bacteria can change with effect of nutritional
conditions, penicillin or lysozyme, and can be cell wall–
deficient
L-FORM: Cell wall partially (spheroplast) or completely
(protoplast) removed bacterial cell.

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• They are capable of normal growth, but do not have cell
walls.
• Under the microscope, oval, star, ring, disc can seen
• L-form can be seen in both Gram-positive and Gram-
negative.
• The cell wall was partially or completely lost.
• L forms are multiform and sensitive to osmotic shock.

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Capsule
• 0.2µm thick viscus layer attached to the cell wall of some
bacteria.
• If capsule is too thick it is known as slime.
• Slime layer are loosely attached to cell wall and can be
lost on strong washing
• Capsule and slime layer are sometimes summarized
under the term glycocalyx.

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• Composition of capsule: 98% water and 2%
polysaccharide or glycoprotein/ polypeptide or both.
• Generally polysaccharide (ex.Streptococcus pneumoniae)
• The capsule of Bacillus anthracis is the polypeptide.

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• Not necessary for the presence of bacterial cells.
• When the capsule disappears spontaneously by
mutation, the bacterium continues its viability.
• Prevents the phagocytosis of bacteria. When the
bacterium loses its capsule, become avirulan and
easily phagocytized
• Capsules helps the organism to adhere to host cells.
• Protects from drying

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• To determine the presence of the capsule, in the prepared
negative preparation (negative staining), the capsule gap
can be determined.
• Streptococcus pneumoniae, Klebsiella pneumoniae
Haemophilus influenzae, Pseudomonas aeruginosa

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Flagella

• Found in many bacterial species and provides movement

of the bacteria
• Especially in rod-shaped bacteria and some spiral shaped
bacteria.
• Cocci-like bacteria are immobile because they don't have
flagella.

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• Bacterial flagella are long, thin (about 20 nm)
• Flagella are free at one end and attached to the cell at
the other end.
• The parts of flagella are the filament, hook and the
basal body.

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• Filament is external to cell wall and is connected to
the hook at cell surface, the hook and basal body
are embedded in the cell wall.
• Hook & filament is composed of protein subunits
called as flagellin. Flagellin is synthesized within the
cell and passes through the centre of flagella.

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• Can never be seen directly with the light microscope but
only after staining with special flagella stains that increase
their diameter.

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Bacteria are divided into five groups according to the position
of the flagella.
Atrichous: without flagella
e.g. Shigella
Monotrichous: With one flagella on one end
e.g. V.cholerae
Lophotrichous: Tuft of flagella at one or both ends
e.g. Stenotrophomonas maltophilia
Amphitrichous: Single flagellum at both ends
e.g. Alcaligenes faecalis
Peritrichous: Flagella surrounding the bacterial cell. All the
members of family Enterobacteriaceae
e.g. Salmonella typhi, Escherichia coli, Proteus spp (highly
motile organism; shows swarming motility) 49
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 Pilus (Fimbria)
• Short, hair-like protein structures
on the surface of prokaryotic
cells.
• Can only be seen in electron
microscopy
• They are thinner, shorter and
more numerous than flagella and
they do not function in motility.
• The fimbriae is composed of a
subunit called pilin

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Simple pilus (100-200 in a cell)
Allows to adhere to surfaces
Thus, it plays a role in bacterial
virulence.
Sex Pilus (1-4 in a cell) (F pilus)
Provides gene transfer between
matched bacteria of DNA

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 Bacterial spore
• Live but dormant state of bacteria.
• Produced during unfavorable condition by the process
called sporulation
• Resistant to nutrition starvation, temperature, extreme pH,
antibiotics etc
• Bacteria are transformed into spore form and protected
against adverse conditions.

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• Spore is not stained by simple or Gram staining;
Stained with special methods
• The place and morphology of spore in the main cell is
specific.
Central (middle)
Terminal (at one end)
Subterminal (close to one end)

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Can be oval or spherical shaped
• Clostridium tetani spores spherical and terminal
• Bacillus anthracis spores are oval and central

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• Depending on the type of bacteria, spores are divided
into two: endospores, exospores
• Especially endospores are characterised by a complex
structure and resistance to high temperatures.
• That is why this dormant form is capable of surviving for
periods longer than thousand years.

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An endospore has layers:

1.Exosporium: it is the outermost

layer made up of protein that
encloses spore coat.
2.Spore coat: It is thick double
layered covering that encloses cortex
3.Cortex: Inside the spore coat, there
is cortex made up of loosely arranged
peptidoglycan layer.
4. Core: It is the innermost part of
spore, It is also known as spore
protoplast

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• Spore formation from a vegetative cell is called
sporulation.
• Spore starts with the doubling of nutrients in the bacteria
and it is completed within 6-8 hours.
• During sporulation, the cell loses most of its water
(contains 5-10% water).
• The low water rate of the spore makes it resistant to high
temperature.

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Components of Bacterial
Spores:

• Thick keratin like coat

• Peptidoglycan
• Cell membrane
• A small amount of cytoplasm
• Very little water
• Bacterial DNA What Makes an Endospore so Resistant?
• Calcium (complexed with dipicolinic acid)
• Small, acid-soluble, DNA-binding proteins
(SASPs)
• Dehydrated core
• Spore coat and exosporium protect
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• When appropriate conditions occur, the formation of
vegetative cells from spore is called germination.
• Germination starts with the deterioration of the most
outer layer of spore
• The process is completed in 90 minutes.

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