This document provides information on bacterial pathogenesis and virulence factors. It begins with definitions of key terms like pathogens, pathogenicity, virulence, and opportunistic pathogens. It then discusses the difference between infection and disease. Koch's postulates for establishing causation are outlined. The document explores factors that influence pathogenicity like virulence factors, number of initial organisms, and immune status of the host. Several virulence factors are described in detail, including toxins, invasiveness through adhesion and penetration of tissues, capsules that aid evading phagocytosis, siderophores for competing with the host for iron, and enzymes that can aid spreading or hydrolyze immunoglobulins.

1. ‫بسم ا الرحمن الرحيم‬
BACTERIAL
PATHOGENESIS
Prof. Khalifa Sifaw Ghenghesh
Dept. of Medical Microbiology,
Faculty of Medicine, Tripoli University,
Tripoli-Libya

2. 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.

3. 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.

4. Koch's Postulates
Isolated
• diseased not healthy people
Growth
• pure culture
Induce disease
• susceptible animals
Re-isolated
• susceptible animals

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

6. VIRULENCE FACTORS

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

8. Toxigenicity. 1
Tissue injury:
• Exotoxins:
• Include several types of protein toxins and
enzymes produced and/or secreted from
pathogenic bacteria.
• Include cytotoxins, neurotoxins, and
enterotoxins.

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

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

11. Invasiveness. 2
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.

12. Penetration and spread
Epithelium
Salmonella typhi
Salmonella enteritidis
Vibrio cholerae

13. 3. Capsules
Antiphagocytic structures
Polysacchride

14. Adhesion. 4
The Relatively Stable, Irreversible
Attachment of Bacteria to a Surface.
• Fimbrial Adhesins
• Nonfimbrial Proteinaceous Adhesins.

15. BACTERIUM
adhesin
receptor
EPITHELIUM

16. E. coli with fimbriae (Pili)

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

18. 6. Other Aggressions
Mainly Enzymes:
•
•
•
•
•
Hyaluronidase >> Spreading Factor.
Coagualse >> Thrombin - Like Enzyme.
Fibrinolysin >> Streptokinase.
Proteases >> Hydrolyse Immunoglobulins.
Others