3. BACTERIAL CELL:
A typical bacterial cell consists of a compound membrane within
which lies the protoplast.
The bacterial cell consists of three layers:
1. Structure external to the cell wall
2. Structure internal to the cell wall
3. Cell wall
4.
5. STRUCTURE EXTERNAL TO THE CELL WALL:
FLAGELLA
FIMBRIAE &
PILI
CAPSULES
SHEATHS
PROSTHECAE
& STALKS
6. • A motile bacterium may
possess a flagellum
(Plural-Flagella).
• The flagellum is hair
like, helical appendages
that protrude through
the cell wall
• And it is responsible for
swimming motility.
7.
8. STRUCTURE OF FLAGELLA:
The structure and function of bacterial flagella have been described
by Simon et al. in 1978.
A flagellum consists of three basic parts:
FILAMENTHOOKBASAL
BODY
9.
10. BASAL BODY:
It attaches the flagellum to the cell wall and plasma membrane.
It is composed of a small central region inserted into a series of rings.
In Gram-negative bacteria two pairs of rings, the Proximal ring and
Distal ring are connected by a central rod.
These two pairs of rings i.e. four rings are:
1. L - (Lipopolysaccharide) ring
2. P - (Peptidoglycan) ring
3. S - (Super membrane) ring
4. M - (Membrane) ring
11. The outer pair of rings, L-ring and P-ring are attached to respective
polysaccharide and peptidoglycan layer of cell wall.
The inner pair of rings, S-ring and M-ring are attached with cell
membrane.
In Gram-positive bacteria only the Distal pair of ring is present.
The S-ring is attached to inside thick layer of peptidoglycan and M-
ring attached to cell membrane.
12. HOOK
It is present outside the cell wall
and connects filament to the basal
body.
It consists of different proteins.
The hook in Gram-positive
bacteria is slightly longer than the
Gram-negative bacteria.
13. FILAMENT OR
SHAFT
• The outermost long region of the flagellum
is called Filament or shaft.
• It has a constant diameter and is made up
of globular proteins, the flagellin.
• The proteins of flagella act to identify
certain pathogenic bacteria.
• Unlike eukaryotes, the filaments are not
covered by a membrane or sheath.
15. Fimbriae:
-Pili and Fimbriae are hair like
appendages bound on surface of cell wall in
Gram-negative bacteria.
-Example: Enterobacteriaceae,
Pseudomondaceae and Caulobacter.
-Eukaryotic cell lacks pili.
-The term fimbriae is used for all
hair like structure covering the surface of
the cell.
16. PILI:
It is genetically governed by Plasmids, the number at which varies
from 3 to 5. It is around 1,000.
It differ from flagella in being shorter and thinner, straight and less
rigid.
But they are in large number.
They occur either at the poles of bacterial cell or evenly distributed
over the enteric surface of the cell.
The pili are 0.2 – 20 μm long with a diameter of about 250A֯.
17. CLASSES OF PILI:
According to the function pili are of two types:
• Common pili which act to adhere the cell to
surfaces.
• Sex pili which join the other bacterial cell for
transfer of genome.
18. STRUCTURE OF PILI:
• Both fimbriae and pili are like flagella as both are the
appendages on the bacterial cell wall.
• They originate from cytoplasm that protrudes outside after
penetrating the peptidoglycan layer of cell wall.
• Fimbriae are made up of 100% protein called Fimbrilin or
pilin which consists of about 163 amino acids.
• Fimbrilin has a molecular weight of about 16,000 Daltons.
• The sex pili are helical tubules consisting of a hollow core
(25-30 A ֯).
• Its filamentous structure is governed by the sex factor
(plasmid) of the bacterium.
• Example: R factor, Col I factor.
19. FUNCTION OF PILI:
Bacteria containing fimbriae are called fimbriate bacteria.
Fimbriae have the adhesive properties which attach the organism to
the natural substrate or to the other organism.
Fimbriae agglutinate the blood cells such as erythrocytes, leucocytes,
epithelial cells, etc.
Fimbriae are equipped with antigenic properties as they act as
thermolabile nonspecific agglutinogen.
Fimbriae affect the metabolic activity.
The sex pili make contact between two cells. They possess hollow
core, they act as conjugation tube.
20. CAPSULE
Some of the bacterial cell are surrounded
by the extracellular polymeric substances
which are commonly called capsule
glycocalyx.
It forms an envelope around the cell wall
and can be observed under light
microscope after special staining
technique called capsular or negative
staining.
It is gelatinous polymer made up of either
polysaccharide (Klebsiella pneumoniae)
or polypeptide (B. anthracis).
21. Polysaccharide may be
a single type of sugars
or several type of
sugars.
Homopolysaccharide Heteropolysaccharide
22. Heteropolysaccharide is synthesized by sugar precursors within the
cell.
Homopolysaccharide constitute the capsule of Acetobacter xylinum.
The capsule of Pneumococci is made up of hexoses, uronic acids and
amino sugars.
The capsule of Streptococci consists of L-amino acids.
If the substances are unorganized and loosely attached to the cell wall,
the capsule is called Slime layer.
The fresh water and marine bacteria trichomes which are enclosed
inside the gelatinous matrix called Sheath.
Sheath is also found in Cyanobacteria or other algae.
23. FUNCTION OF CAPSULE:
• It may prevent the attachment of bacteriophage.
• It protects the bacterial cells against dessication as it is hygroscopic
and contains water molecules.
• It may survive in natural environment due to its sticky property.
• Example: Plant root surfaces, human teeth, tissues, etc.
• It protects from phagocytosis.
• S.mutans uses its capsules as a source of energy.
25. PROSTHECAE:
Prosthecae (singular-prostheca) are semirigid extensions of the cell
wall and cytoplasmic membrane and have a diameter that is always
less than that of the cell.
They are characteristic of a number of aerobic bacteria from fresh
water and marine environments.
Some bacterial genera such as Caulobacter have a single prostheca,
others such as Stella and Ancalomicrobium have several.
It increase the surface area of the cells for nutrient absorption.
26. STALK:
The term stalk is sometimes used interchangeably with prostheca or
hypha.
It is better to restrict its use to certain non living ribbon like or tubular
appendages that are excreted by the cell.
Example: Gallionella or Planctomyces.
These stalks aid in attachment of the cells to surfaces.
30. • Beneath the cell wall is the cytoplasmic membrane.
• Its structure is approximately 7.5nm thick.
• Its is composed primarily of phospholipids (about 20 to 30% ) and
proteins (60 to 70%).
31. PHOSPHOLIPIDS:
It forms a bilayer in which most if the proteins are tenaciously held (Integral
proteins).
These proteins can be removed by destruction of the membrane, as with
treatment of detergents.
Other proteins are only loosely attached (Peripheral proteins) and can be
removed by mild treatments such as osmotic shock.
The lipid matrix of the membrane has fluidity, allowing the components to
move around literally.
In Eubacteria, it is phosphoglycerides, in which straight – chain fatty acids are
ester – linked to glycerol.
In Archaeobacteria, the lipids are polyisopyrenoid branched-chain lipids, in
which long-chain branched alcohols (phytanols) are ester-linked to glycerol.
32. It is a hydrophobic barrier to penetration by most water-soluble
molecules.
However, specific proteins in the membrane allow, indeed facilitate,
the passage of small molecules (i.e., nutrients & waste products)
across the membrane.
It also contains various enzymes involved in respiratory metabolism
and in synthesis of capsular and cell wall components.
33. PROTEINS:
• It is synthesized within the cell, but some can pass across the
cytoplasmic barrier to the outside.
• Example: Porins or Lipoproteins.
• The extracellular enzymes that are secreted by many bacteria into their
culture medium, such as Penicillinases, Proteinases and Amylases.
• Special membrane proteins might bind the signal peptide at the inner
surface of the cytoplasmic membrane.
• And form a channel by which the protein can traverse the membrane.
35. PROTOPLASTS:
• It is portion of a bacterial cell consisting of the cytoplasmic membrane and
the cell material bounded by it.
• It can be prepared from Gram-positive bacteria by treating the cells with a n
enzyme such as lysozyme, which selectively dissolves the cell wall.
• It may culture the bacteria by the presence of an antibiotic such as
Penicillin, which prevents the formation of the cell wall.
• Bacteria normally occur in hypotonic environments (lower osmotic
pressure) and they continuously take up water by osmosis.
• Thus, they tend to expand, pressing the cytoplasmic membrane tightly
against the rigid cell wall.
36. SPHEROPLASTS:
The cytoplasmic membrane of the protoplasts plus the outer
membrane of the cell wall, the cell is called spheroplast rather than a
protoplast.
Some bacteria, the mycoplasmas, never have cell walls and are
bounded by only a cytoplasmic membrane.
Most mycoplasmas are parasites of animals, plants or arthropods and
live in osmotically favorable or isotonic environments.
Some are able to attain a degree of rigidity by incorporating
cholesterol into their cytoplasmic membranes.
38. Bacteria cells do not contain membrane-enclosed organelles
corresponding to the mitochondria and chloroplasts of eukaryotic
cells.
Gram-positive bacteria, possess membrane invaginations in the form
of systems of convoluted tubules and vesicles termed Mesosomes.
Those known as central mesosomes penetrate deeply into the
cytoplasm, are located near the middle of the cell, and seen to be
attached to the cell’s nuclear material.
They are thought to be involved in DNA replication and cell division.
39. Peripheral mesosomes show only a shallow penetration into the
cytoplasm, are not restricted to a central location , and are not
associated with nuclear material.
They seem to be involved in export of exocellular enzymes such as
Penicillinase.
Extensive intracellular membrane system occur in methane-oxidising
bacteria, in certain chemoautotrophic bacteria.
In the phototrophs known as cyanobacteria, special intracellular
membrane (thylakoids) occur that seem to be separate from the
cytoplasmic membrane.
41. The cell material bounded by the cytoplasmic membrane may be
divided into:
1. The cytoplasmic area, granular in appearance and rich in the
macromolecular RNA-protein bodies known as ribosomes, on which
proteins are synthesized.
2. The chromatinic area, rich in DNA.
3. The fluid portion with dissolved substances.
42. Unlike animal or plant cells, there is no endoplasmic reticulum to which
ribosomes are bound,.
Some ribosomes are free in the cytoplasm.
Especially those involved in the synthesis of proteins to be transported out of the
cell, are associated with the inner surface of the cytoplasmic membrane.
When the ribosomes of prokaryotes undergo sedimentation in a centrifuge, they
have a sedimentation coefficient of 70 Svedberg units (70s).
And are composed of two subunits, 50s and 30s subunit.
This is in contrast to the ribosome of eukaryotic organisms, which have a
sedimentation coefficient of 80s and are composed of 60s and 40s subunit.
44. Volutin granules, also known as metachromatic granules are
composed of polyphosphate.
In aerobic bacteria, especially under high-carbon, low-nitrogen
culture conditions, is a chloroform-soluble, lipid like material, poly-β-
hydroxybutyrate (PHB), which can serve as a reserve carbon source.
Polysaccharide granules (glycogen) can be stained brown with iodine.
Another type of inclusion is represented by the intracellular globules
of elemental sulphur that may inoculate in certain bacteria growing in
environments rich in hydrogen sulphide.
45. Some bacteria that live in aquatic habitats form gas vacuoles that
provide buoyancy.
By light microscopy they are bright, refractile bodies.
By electron microscopy they are seen to have a regular shape: hollow,
rigid cylinders with more or less conical ends and having a striated
protein boundary.
The identifying feature of gas vacuoles is that they can be made to
collapse under pressure and there by lose their refractility.
47. In eukaryotic cells, bacterial cells contain neither a distinct
membrane-enclosed nucleus nor a mitotic apparatus.
They do not contain an area near the centre of the cell that is regarded
as a nuclear structure, and DNA of the cell is confined to this area.
Because it is not a discrete nucleus, this nebulous has been designated
by such terms as the nucleoid, the chromatin body, the nuclear
equivalent, and even the bacterial chromosome.
Since it consists of a single, circular DNA molecule in which all the
genes are linked.
48. The nucleoid can be made visible under the light microscope by
Feulgen staining, which is specific for DNA.
The behaviour for nucleoid in growing, dividing bacteria has been
observed by use of phase-contrast microscope with a medium having
a high refractive index.
50. Beneath the external structures as capsules, sheaths and flagella and
external to the cytoplasmic membrane is the cell wall.
PEPTIDOGLYCAN:
Peptidoglycan (murein) an insoluble, porous, cross-linked polymer of
enormous strength and rigidity.
It is found only in prokaryotes, it occurs in the form of a ‘‘bag-shaped
macromolecules’’ surrounding the cytoplasmic membrane.
It differs in composition and structure from one species to another but
it is basically a polymer of N-acetylglucosamine, N-acetyl muramic
acid, L-alanine, D-alanine, D-glutamate and Diamino acid.
Portions of the peptidoglycan must continually be degrade by wall-
associated hydrolytic enzymes. So, that new polymer can be added.
52. WALLS OF GRAM-POSITIVE EUBACTERIA
It usually have a much greater amount of peptidoglycan in their cell
walls than do Gram-negative bacteria.
It may account for 50 percent or more of the dry weight of the wall of
some Gram-positive species.
But only about 10 percent of the wall of Gram-negative bacteria.
Other substances may occur in addition to peptidoglycan.
The walls of Streptococcus pyogenes contain polysaccharide that are
covalently linked to the peptidoglycan and which can be extracted
with hot dilute hydrochloric acid.
53.
54. The walls of Staphylococcus aureus and Staphylococcus faecalis
contain teichoic acids-acidic polymers of ribitol phosphate or glycerol
phosphate.
They are covalently linked to peptidoglycan and which can be
extracted with cold dilute acid.
55. WALLS OF GRAM-NEGATIVE BACTERIA
It is more complex than those of Gram-positive bacteria.
The difference is the presence of an outer membrane that surrounds a
thin underlying layer of peptidoglycan.
Because of this membrane, the walls of Gram-negative bacteria are
rich in lipids (11 to 22 percent of the dry weight of the wall).
This outer membrane serves as an impermeable barrier to prevent the
escape of important enzymes, such as those involved in cell wall
growth, from the space between the cytoplasmic membrane and the
outer membrane (periplasmic space).
56. The outer membrane also serves as a barrier to various external
chemicals and enzymes that could damage the cell.
The outer membrane of the Gram-negative cell wall is anchored to the
underlying peptidoglycan by means of Braun’s lipoprotein.
The membrane is a bilayered structure consisting mainly of
phospholipids, proteins, and lipopolysaccharide (LPS).
The LPS has toxic properties and is known as endotoxin.
57. It occurs only in the outer layer of the membrane and is composed of
three covalently linked parts.
LIPID A, firmly embedded in the membrane.
CORE POLYSACCHARIDE, located at the membrane surface.
POLYSACCHARIDE O ANTIGENS, which extend like whiskers
from the membrane surface into the surrounding medium.
58. REFERENCES
1. Michael J. Pelczar, JR. A Textbook of
Microbiology, Sixth edition, Page no: 78-94.
2. Dr. R.C. Dubey, A Textbook of
Microbiology, S. Chand Publication, Second
Edition, Page no: 80-96.
