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

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

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

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

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

Virulence factors help bacteria to:
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Invade the host,
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Cause disease, and
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Evade host defenses.

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

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

Differentiation of Exotoxins and Endotoxins.
Exotoxins Endotoxins
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-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.
-Do not produce fever in host.
Weakly toxic.
Often produce fever in host.

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.

Epithelium
Salmonella typhi
Salmonella enteritidis
Vibrio cholerae

Antiphagocytic structures
Polysacchride

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

adhesin
BACTERIUM
EPITHELIUM receptor

E. coli with fimbriae (Pili)

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

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