Prof. Khalifa Sifaw Ghenghesh

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??? ???? ?????? ??????BACTERIAL PATHOGENESIS

• Prof. Khalifa Sifaw Ghenghesh
• Dept. of Medical Microbiology,
• Faculty of Medicine, Tripoli University,
• Tripoli-Libya

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• The Pathogen
• A Disease Producing Microorganism.
• Pathogenicity
• Capacity to Initiate Disease.
• Virulence
• Capacity to Harm the Host.
• Refer to Degree of Pathogenicity.
• Opportunistic Pathogens
• Common or Non Pathogenic Microbes. e.g. Normal
  Flora of the Body.

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• Infection
• The Lodgement and Multiplication of a Parasite in
  or on the Tissues of a Host.
• Disease
• A Rare Consequence of Infection.
• Measurement of Virulence
• The Median Lethal Dose (LD50) is Used.
• Number of Microorganisms or Micrograms of Toxin
  Required to Kill 50 of Infected Animals.

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Koch's Postulates

• Isolated
• diseased not healthy people
• Growth
• pure culture
• Induce disease
• susceptible animals
• Re-isolated
• susceptible animals

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PATHOGENICITY

• Virulence factors (Determinants of pathogenicity)
• Number of initial organisms
• Immune status

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VIRULENCE FACTORS
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• Virulence factors help bacteria to
• Invade the host,
• Cause disease, and
• Evade host defenses.

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1. Toxigenicity

• Tissue injury
• Exotoxins
• Include several types of protein toxins and
  enzymes produced and/or secreted from pathogenic
  bacteria.
• Include cytotoxins, neurotoxins, and enterotoxins.

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• Endotoxins
• Lipopolysaccharide
• Gram-negative bacteria
• Endotoxic (Septic) Shock
• Hypotension (tissue pooling of fluids)
• Disseminated intravascular coagulation
• Fever
• Lack of effective oxygenation
• Overall system failure

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Differentiation of Exotoxins and Endotoxins.
Exotoxins Endotoxins -------------
--------------------------------------------------
------------------------ -Excreted by living
cells. -Released after death of
bacteria. -Relatively unstable. -Relatively
stable. -Highly antigenic -Do not stimulate
formation stimulate the formation of
antitoxin. of high-titer antitoxin. -Converted
into antigenic, -Not converted into toxoids.
nontoxic toxoids. -Highly toxic. -Weakly
toxic. -Do not produce -Often produce fever
fever in host. in host.
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2. Invasiveness

• Capacity of a pathogen to spread in the host
  tissues after establishing infection.
• Surface components that allow the bacterium to
  invade host cells can be encoded on plasmids, but
  more often are on the chromosome.

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Penetration and spread
Epithelium
Salmonella typhi
Salmonella enteritidis
Vibrio cholerae
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3. Capsules

• Antiphagocytic structures
• Polysacchride

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4. Adhesion

• The Relatively Stable, Irreversible Attachment of
  Bacteria to a Surface.
• Fimbrial Adhesins
• Nonfimbrial Proteinaceous Adhesins.

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BACTERIUM
adhesin
receptor
EPITHELIUM
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E. coli with fimbriae (Pili)
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5. Siderophores

• Iron-binding factors that allow some bacteria to
  compete with the host for iron, which is bound to
  hemoglobin, transferrin, and lactoferrin.

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6. Other Aggressions

• Mainly Enzymes
• Hyaluronidase gtgt Spreading Factor.
• Coagualse gtgt Thrombin - Like Enzyme.
• Fibrinolysin gtgt Streptokinase.
• Proteases gtgt Hydrolyse Immunoglobulins.
• Others