Download Pathogenesis

Bacterial Pathogenesis
Outline
Bacterial Pathogenesis
•
•
•
•
Introduction
Host Susceptibility
Pathogenic Mechanisms
Virulence Factors
Introduction of Bacterial
Pathogenesis
• Infection: growth and multiplication of a microbe in or on
the body with or without the production of disease.
• The capacity of a bacterium to cause disease reflects
Its relative “Pathogenicity.”
• Virulence is the measure of the pathogenicity of a
microorganism.
• Pathogenesis refers both to the mechanism of infection
and to the mechanism by which disease develops.
Host Susceptibility
• Susceptibility to bacterial infections
=>Host Defenses vs Bacterial Virulence
• Host Defenses:
Barriers (skin & mucus)–first line
Innate Immune Responses (complement,
macrophages & cytokines)–the early stage
Adaptive Immune Responses (Ag-specific B
& T cells)–the later stage
• Host defenses can be comprised by
destructing barriers or Defective immune
response.
Pathogens
• Strict pathogens
Are more virulent and can cause diseases in a
normal person.
• Opportunistic pathogens
Are typically members of normal flora and
cause diseases when they are introduced into
unprotected sites, usually occur in people with
underlying conditions.
Entry into the human body
• The most frequent portals of entry
- Mucus
- -Skin
• Routes:
Ingestion, inhalation,
trauma ,needles,
catheters,
arthropod bite,
sexual transmission
Characteristics of Pathogenic Bacteria
1. Transmissibility
2. Adherence to host cells
3. Invasion of host cells and tissue
4. Evasion of the host immune system
5. Toxigenicity
Abacterium may cause diseases by
1. Destroying tissue (invasiveness)
2. Producing toxins (toxigenicity)
3. Stimulating overwhelming host immune
responses
Pathological Mechanism of Bacterial
Infections
• Bacteria-mediated
Pathogenesis
• Host-mediated
Pathogenesis
• Bacterial virulence
Factors
=>bacterial factors
causing diseases
Bacterial virulence Mechanisms
• Adherence
• Invasion
•
•
•
•
•
•
•
•
•
•
By products of growth (gas,acid)
Toxins
Degradative enzymes
Cytotoxic protiens
Endotoxin
Superantigen
Induction of excess inflammation
Evasion of phagocytic and immune clearance
Capsule
Resistance to antibiotics
Virulence Factors
virulence factors help bacteria to
• (1) invade the host (2) cause disease (3) evade
host defenses. The following are types of
virulence factors:
• Adherence Factors: Many pathogenic bacteria
colonize mucosal sites by using pili (fimbriae) to
adhere to cells.
• Invasion Factors: Surface components that allow
the bacterium to invade host cells can be
encoded on plasmids, but more often are on the
chromosome.
• Capsules: Many bacteria are surrounded by capsules
that protect them from opsonization and phagocytosis.
• Endotoxins: The lipopolysaccharide endotoxins on
Gram-negative bacteria cause fever, changes in blood
pressure, inflammation, lethal shock, and many other
toxic events.
• Exotoxins: Exotoxins include several types of protein
toxins and enzymes produced and/or secreted from
pathogenic bacteria. Major categories include
cytotoxins, neurotoxins, and enterotoxins.
• Siderophores: Siderophores are iron-binding factors
that allow some bacteria to compete with the host for
iron, which is bound to hemoglobin, transferrin, and
lactoferrin
Adhesion
• Adherence of bacterium to epithelial or
endothelial Cells allow them to colonize the
tissue.
• Common
adhesins:
pili(fimbriae),
slime,
lipoteichoicacid, surface proteins or lectins.
• Biofilm , formed on a surface by the bacteria that
are bound together within a sticky web of
polysaccharide, is a special bacterial adaptation
that facilitates colonization on the surgical
appliances (e.g. artificial valves or indwelling
catheters) and dental plaque. It can protect the
bacteria from host defenses and antibiotics.
The bacteria may invade via the M cells