MICROBIAL MECHANISMS OF PATHOGENICITY'

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MICROBIAL MECHANISMS OF PATHOGENICITY.

• Introduction How microorganisms enter a host.

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MECHANISMS OF PATHOGENICITY.
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PORTALS OF ENTRY.

• Mucous membranes.
• Skin.
• Parenteral Route.
• The Preferred Portal of Entry.

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NUMBERS OF INVADING MICROBES.

• The virulence of a microbe or the potency of its
  toxin is often expressed as the LD50.
• LD50(lethal dose for 50 of hosts) is the the
  number of microbes in a dose that will kill 50
  of inoculated test animals.
• ID50(infectious dose for 50 of hosts) is the
  dose required to produce a demonstrable infection
  in 50 of the test animals.

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ADHERENCE.

• Means attachment.
• A necessary step in pathogenicity.
• Attachment between pathogen and host is
  accomplished by means of adhesins or ligands.
• Most adhesins of microbes are glycoproteins or
  lipoproteins.

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ADHESINS ARE VERY DIVERSE.

• S. mutans plays a key role in tooth decay
  attaches to the surface of teeth by its
  glycocalyx.
• E. coli have adhesins on fimbriae that adhere
  only to specific kinds of cells.
• N ghonorrhoeae has fimbriae containing adhesins
  which permit attachment to cells with appropriate
  receptors in the G.I tract.

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HOW BACTERIA PENETRATE HOST DEFENSE

• Capsules.
• Components of the cell wall
• M protein.
• Enzymes
• Hemolysins.
• Coagulases.
• Hyaluronidase.

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• Collagenase.
• Necrotizing factors.
• Hypothermic factors.
• Lecithinase.

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HOW BACTERIA DAMAGE HOST CELLS.

• Direct damage.
• The production of Toxins Toxin, Toxigenicity,
  Toxemia.
• Types of toxins Exotoxins and Endotoxins.

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EXO and ENDOTOXINS.
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ACTION OF AN EXOTOXIN.
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EXOTOXINS.

• Produced inside some bacteria as part of their
  growth and metabolism and released into the
  surrounding medium.
• Are proteins, and many are enzymes.
• Most bacteria that produce exotoxins are
  gram-positive.
• The genes for most exotoxins are carried on
  bacterial plasmids or phages.

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EXOTOXINS.

• Are soluble in body fluids, so can easily diffuse
  into the blood and are rapidly transported
  throughout the body.
• Work by destroying particular parts of the hosts
  cells or by inhibiting certain metabolic
  functions.

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EXOTOXINS.

• Three principal types
• Cytotoxins- kill host cells or affect their
  functions, e.g. Corynebacterium diphtheriae
• This type of exotoxin binds to eF(elongation
  factors) in protein synthesis in the epithelial
  cells of the throat.

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Neurotoxin.

• Target the nervous system, and can interfere with
  normal nerve impulse transmission, e.g. C.
  tetani, C. botulinum.

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ENTEROTOXINS.

• Affect cells lining the gastrointestinal tract.
• E.g. V. cholerae, C. difficile.

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EXOTOXINS.

• Are among the most lethal substances known.
• Only 1mg of the botulinum exotoxin is enough to
  kill 1 million guinea pigs.
• Are disease-specific.
• Body produces antitoxins that provide immunity to
  exotoxins.

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NOTABLE EXOTOXINS.

• Diphtheria toxin.
• Erythrogenic toxins.
• Botulinum toxin.
• Tetanus toxin
• Vibrio Enterotoxin.
• Staphylococcal Enterotoxin.
• .
• .

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ENDOTOXINS

• Part of the outer portion of the cellwall of gram
  negative bacteria.
• Gram negative outer membrane consists of
  lipoproteins, phospholipids, and
  liposaccharides(LPS).
• Lipid portion of LPS, called lipid A is the
  endotoxin.
• Are liposaccharides.

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• Exert their effects when the gram negative
  bacteria dies and their cell wall undergo lysis,
  thus liberating the endotoxin(e.g use of
  antibiotics).
• All endotoxins produce the same signs and
  symptoms.
• Endotoxins can also induce miscarriage.
• Can activate blood-clotting proteins(clots).

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• Shock.
• Septic shock.
• Organisms that produce endotoxin include
  Salmonella typhi, proteus species, Neisseria
  meningitidis.

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CPE EFFECTS OF VIRUSES.

• CPE.
• INCLUSION BODIES.
• SYNCYTIA or GIANT CELLS.