Prof. Khalifa Sifaw GhengheshDept. of Medical Microbiology,

Faculty of Medicine, Tripoli University,

Tripoli-Libya

الرحيم الرحمن الله بسم

BACTERIAL PATHOGENESIS

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.

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.

Koch's Postulates

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

PATHOGENICITY

VIRULENCE FACTORS

Virulence factors help bacteria to: • Invade the host,

• Cause disease, and

• Evade host defenses.

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.

• Endotoxins:• Lipopolysaccharide • Gram-negative bacteria• Endotoxic (Septic) Shock:

• Hypotension (tissue pooling of fluids)• Disseminated intravascular coagulation• Fever• Lack of effective oxygenation• Overall system failure

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.

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.

Penetration and spread

Vibrio choleraeVibrio cholerae

SalmonellaSalmonella enteritidisenteritidis

SalmonellaSalmonella typhityphi

EpitheliumEpithelium

3. Capsules

Antiphagocytic structures

Polysacchride

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

• Nonfimbrial Proteinaceous Adhesins.

4 .Adhesion

adhesinadhesin

EPITHELIUMEPITHELIUM

receptorreceptor

BACTERIUMBACTERIUM

E. coli with fimbriae (Pili)

5. Siderophores

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

6. Other Aggressions

Mainly Enzymes:• Hyaluronidase >> Spreading Factor.

• Coagualse >> Thrombin - Like Enzyme.

• Fibrinolysin >> Streptokinase.

• Proteases >> Hydrolyse Immunoglobulins.• Others