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7/28/2019 Principles of Microbial Pathogenicity
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Introduction:
Pathogenicity :
Is the ability to cause disease by overcoming the defences of
a host
Virulence : Is the degree of pathogenicity.
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How Microorganisms Enter a HostThe specific rout by which a particular pathogen gains access
to the body is called its portal of entry.
Portal of Entry:Many of microorganisms can penetrate Mucous
membranes of the Conjunctiva and the Respiratory(Microorganisms that are inhaled with droplets of moisture and
dust particles gain access to the respiratory tract. And It is the
most common portal of entry).
Gastrointestinal (Microorganisms enter the gastrointestinal
tract via food, water, and contaminated fingers).
Genitourinary tracts (Microorganisms that gain access via
the genitourinary tract can enter the body through mucous
membranes).
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Most microorganisms
cannot penetrate intact
skin; they enter hair
follicles and sweatducts.
Some microorganisms
can gain access to
tissues by inoculation
through the skin and
mucous membranes in
bites , injections , andother wounds. This
rout of penetration is
called the parenteral
route.
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The Underlying Mechanisms of BacterialPathogenicity
Two broad qualities of pathogenic bacteria underlie the means
by which they cause disease:
1. Invasiveness is the ability to invade tissues. It
encompasses mechanisms forcolonization (adherence and
initial multiplication), production of extracellular substances
which facilitate invasion (invasions) and ability to bypass
or overcome host defence mechanisms.
2.Toxigenesis is the ability to produce toxins. Bacteria may
produce two types of toxins called exotoxins and endotoxins.
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Colonization:The first stage ofmicrobial infection is
colonization: the
establishment of thepathogen at the
appropriate portal of
entry. Pathogens usually
colonize host tissues thatare in contact with the
external environment
Sites of entry in human
hosts include the
urogenital tract, thedigestive tract, the
respiratory tract and the
conjunctiva. Organisms
that infect these regionshave usually developed
tissue adherence
mechanisms and some
ability to overcome orwithstand the constant
pressure of the host
defenses at the surface.
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Mechanisms of
Adherence to Cell or
Tissue Surfaces
The mechanisms for
adherence may involve two
steps:1. nonspecific adherence:
reversible attachment of the
bacterium to the eucaryotic
surface (sometimes called"docking") .
2. specific adherence: reversible
permanent attachment of the
microorganism to the surface
(sometimes called "anchoring").
Specific adherence
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How bacterial pathogens penetrate HostDefenses :
CapsulesSome pathogens have capsules that prevent them from beingphagocytized.
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Cell wall component
Proteins in the cell wall can facilitate adherence orprevent a pathogen from being phagocytized
Enzymes-Local infections can be protected in fibrin clot caused by the
bacterial enzyme coagulase.
-Bacterial can spread from a focal infection by means of
kinases
(which destroy the blood clots) , hyaluronidase (whichdestroys a mucopolysaccharide that holds cellstogether) , and collagenase (which hydrolyzes connectivetissue collagen).-IgA proteases destroy IgA antibodies.
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Antigenic Variation:Some microbes vary expression
of antigens, thus avoiding the
hosts antibodies.
Penetration into the HostCell Cytoskeleton:
Bacterial may produce proteinsthat alter the action of the host
cells cytoskeleton allowing
bacteria into the cell.
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Killing of phagocytes:
An alterative strategy is for the microorganism to kill
the phagocyte, This can be achieved by the
production of leucocidins (e.g. Staphylococci,streptococci ) wich promote the discharge of
lysosomal substances into the cytoplasm of the
phagocyte rather than into the vacuole, thus directing
the phagocytes lethal activity towards itself
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How Bacterial Pathogens DamageHost Cell:
Using the Hosts Nutrients: SiderophoresBacteria get iron from the host using siderophores.
Direct DamageHost cells can be destroyed when pathogens
metabolize and multiply inside the host cells.
The production of Toxins
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References:
Tortora & Funke & Case (2010) Microbiology: AnIntroduction,10th ed., Benjamin Cummings.
Hugo & Russell`s (2004) Pharmaceutical microbiology, 7th
ed., Blackwell Publishing
Todar's Online Textbook of Bacteriology
(http://www.textbookofbacteriology.net/pathogenesis.html).
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