Download Lecture 5 – Prokaryotic cell structures continued

Lecture 5
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Lecture 5 – Prokaryotic cell structures continued
Some plasma membrane invagination lead to particular metabolic
activities in certain bacterial
 Cyanobacteria use internal membranes as a scaffold for
chlorophyll
 Nitrifying bacteria use internal membranes for respiratory
activity
Glycocalyces (singular glycocalyx) - coatings of macromolecules that protect
cell and help adherence, normally associated outside the cell wall. The
glycocalyx is usually a viscous polysaccharide or polypeptide slime. Actual
production of a glycocalyx often depends on environmental conditions. Two
types;
Capsule – this layer is well organized and is
not easily removed from the cell’s exterior
(28,000X)
1. Can be associated with virulence b/c
unable to be phagocytosed; decreases
phagocytosis
a. Virulence = in infection, the
relative capacity of a pathogen to
invade and harm a host
b. Loss of capsule = decreased
virulence
2. Prevents
dehydration/dessication/drying; highly
hydrated
3. Keeps many hydrophobic, toxic cmpds
from entering the cell
4. Can serve as a receptor, allowing
viruses to attach to the cell
5. Usually negatively charged; must use
structural stain (India ink) to view or
viewed under an electron microscope
Example: Streptococcus mutans- forms
dental plaque
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S-layer – structurally different from the capsule and
slime layer. The S-layer has been associated with a
number of possible functions. These include the
following:
a. The S-layer may protect bacteria
from harmful enzymes, from changes
in pH, and from the predatory
bacterium Bdellovibrio, a parasitic
bacterium that actually uses its motility
to penetrate other bacteria and replicate
within their cytoplasm.
b. The S-layer can function as an
adhesion, enabling the bacterium to
adhere to host cells and
environmental surfaces, colonize,
and resist flushing.
c. The S-layer may contribute to
virulence by protecting the bacterium
against complement attack and
phagocytosis.
Pili (s. = pilus) are thin, protein tubes originating from the cytoplasmic membrane
and are found in virtually all gram-negative bacteria but not in many grampositive bacteria. The pilus has a shaft composed of a protein called pilin. At the
end of the shaft is the adhesive tip structure having a shape corresponding
to that of specific glycoprotein or glycolipid receptors on a host cell.
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2 types
Type I - short attachment pili, also known as fimbriae, that are usually quite
numerous
 adhesion allowing bacteria to colonize
environmental surfaces or cells and
resist flushing.
 Because both the bacteria and the host
cells have a negative charge, pili may
enable the bacteria to bind to host cells
without initially having to get close enough to be pushed away by
electrostatic repulsion.
 Once attached to the host cell, the pili can depolymerize and enable
adhesions in the bacterial cell wall to make more intamate contact.
 Bacteria are constantly losing and reforming pili as they grow in the body
and the same bacterium may switch the adhesive tips of the pili in
order to adhere to different types of cells and evade immune defenses.
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And conjugation or sex pilus that enables conjugation.
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conjugation is the transfer of DNA from a donor or male bacterium with a
sex pilus to a recipient or female bacterium to enable genetic
recombination.
Electron Micrograph of Escherichia coli with a Conjugation Pilus
Electron Micrograph of Escherichia coli with Pili
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In order to protect against infection, one of the things the body must
initially do is detect the presence of microorganisms. The body
does this by recognizing molecules unique to microorganisms
that are not associated with human cells. These unique
molecules are called pathogen-associated molecular patterns.
The pilin in bacterial pili binds to pattern-recognition receptors on
a variety of defense cells of the body and triggers innate immune
defenses such as inflammation, fever, and phagocytosis.
Flagella
 Threadlike locomotory appendages; extend outward from plasma
membrane and cell wall
 A bacterial flagellum has 3 basic parts: a filament, a hook, and a basal
body.
The filament is the rigid, helical structure that extends from the cell surface.
 It is composed of the protein flagellin arranged in helical chains so as to
form a hollow core. During synthesis of the flagellar filament, flagellin
molecules coming off of the ribosomes are thought to be transported
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through the hollow core of the filament where they attach to the growing
tip of the filament causing it to lengthen.
With the exception of a few bacteria such as Bdellovibrio and Vibrio
cholerae, the flagellar filament is not surrounded by a sheath

The hook is a flexible coupling between the filament and the basal body
The basal body consists of a rod and a series of rings that anchor the flagellum
to the cell wall and the cytoplasmic membrane.
 Unlike eukaryotic flagella, the bacterial flagellum has no internal fibrils and
does not flex.
 Instead, the basal body acts as a molecular motor, enabling the
flagellum to rotate and propell the bacterium through the surrounding
fluid. In fact, the flagellar motor rotates very rapidly.
Basal Body make up about 1% of total mass of the bacterium. They are thin
& small; composed of protein in series of rings
In Gram negative bacteria it is surrounded by 4 collars/rings:
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
S ring attached to plasma membrane

M ring attached to plasma membrane

P ring anchored to PG

L ring associated with outer
membrane – LPS
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In Gram positive bacteria there are only 2 rings
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inner ring attached to plasma
membrane
 outer ring associated with cell
wall attached to PG
(*rings serve to anchor flagellum to
membrane and wall)
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Bacteria flagella are 10-20 µm long and between 0.01 and 0.02 µm in diameter
and come in a number of distinct arrangements
Flagella vary in both # and arrangement
1. monotrichous: a single flagellum, usually at one pole
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2. amphitrichous: a single flagellum at both ends of the
organism
3. lophotrichous: two or more flagella at one or both poles
4. peritrichous: flagella over the entire surface
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5. axial filaments: internal flagella found only in the
spirochetes. Axial filaments are composed of from two
to over a hundred axial fibrils (or endoflagella) that
extend from both ends of the bacterium between the
outer membrane and the cell wall, often overlapping
in the center of the cell.
A popular theory as to the mechanism behind spirochete
motility presumes that as the endoflagella rotate in the
periplasmic space between the cytoplasmic membrane
and the cell wall, this could cause the corkscrew-shaped
outer membrane of the spirochete to rotate and propell
the bacterium through the surrounding fluid.
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