Transcript Structures external to the Cell Wall:

Structures external to the Cell Wall:
( glycocalyx, flagella, axial filaments, and pili).
1- Glycocalyx and Capsule:
Many bacteria synthesize large amounts of extracellular polymers when
growing in their natural environments.
These polymers form capsules or glycocalyx. Its chemical nature may be
polysaccharide as in the Pneumococcus or polypeptide as in Bacillus
anthracis .
When these polymers closely surrounding the cell; it is called Capsule, But if
these polymers form a loose meshwork of fibrils extending outward from the
cell; it is called glycocalyx
Glycocalyx and Capsule:
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Function of capsule or glycocalyx is to protect bacterial cells from
phagocytosis.
Capsule and glycocalyx can be demonstrated by light microscope using of
capsule stain of bacterial smear
or India ink wet mount.
2-Flagella and motility:
Motile bacteria possess filamentous appendages known as flagella, which
act as organs of locomotion.
The flagellum is a long, thin filament, twisted spirally in an open, regular
waveform.
It is about 0.02 μm thick and is usually several times the length of the
bacterial cell.
According to the species, there may be one, or up to 20, flagella per cell.
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Flagella may be classified according to there arrangement as follow:
1- Monotrichous (single polar flagellum).
2- Lophotrichous (tuft of polar flagella).
3- Perituichous (flagella distributed over the cell).
4- Amphitrichous (one flagellum at each side of cell).
Flagella are associated with chemotaxis process (chemical attraction)
of bacterial cells that contribute in disease pathogenesis.
Monotrichous flagellum
Lophotrichous flagella
Peritrichous flagella
3- Axial filaments:
Some types of bacteria have a flagellum that lie inside periplasmic space
(over cell wall peptidoglycan and under the outer membrane).
This flagellum called endoflagellum or axial filament.
The endoflagella are more than half the length of the organisms and run
along the axial aspect of the spiral body.
They are responsible for rotary motility of these organisms.
4- Pili and Fimbriae:
Many bacteria possess filamentous appendages termed pili or
fimbriae.
These are more numerous than flagella (e.g. 100-500, being
borne surrounding each cell).
They are from 0.1 to 1.5 μm in length and having a uniform
width between 4 and 8 nm.
Pili are important in mediating adhesion between the bacteria
and host cells ( hemagglutination phenomenon).
Another type of pili called sex pili is longer and initiate the
process of conjugation ( genetic material exchange between
bacteria).
Pili and Fimbriae:
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The Bacterial Cell Wall:
The cell envelope:
The layers that surrounding bacterial cell are called the cell envelope.
The Gram-positive cell envelope:
It is composed from three layers: the cytoplasmic membrane, a thick
peptidoglycan layer and a variable outer layer called capsule.
The Gram-negative cell envelope:
It is composed from inner membrane (cytoplasmic membrane), the outer
membrane, the peptidoglycan sheet, and capsules if present.
Periplasmic space separates outer and inner membrane.
The Gram-positive cell envelope:
The Gram-negative cell envelope:
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The cell wall:
It is the outer covering layer of the bacterial cell.
It is a rigid structure consisting of two layers in Gram-positive bacteria and
of three layers in Gram-negative.
In Gram-positive bacteria, it is composed from pepeidoglycan (thick) layer
and the associated capsule if present.
Peptidoglycan is long polymers of two sugar derivatives, NAG (N-Acetyl
Glucosamine) and NAM (N-Acetyl Muramic acid) with side chains of four
alternating D and L amino acids attached to NAM.
The cell wall structure:
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Structures internal to the Cell Wall:
The cytoplasmic membrane:
It is consisting of phospholipids bilayers and proteins (integral and peripheral
protein).
The major functions of
cytoplasmic membrane
are:
1- Selective permeability and
transport of solutes.
2- Electron transport by its
electron transport chain.
3- Excretion of hydrolytic
exoenzymes.