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Mechanisms of Pathogenicity
Microbiology 2314
Definitions
• Pathogenicity
The ability of a pathogen to produce a disease
by overcoming the defenses of the host.
• Virulence
The degree of pathogenicity
• Portal of Entry
The specific route by which a particular
pathogen gains access to the body.
Portals
of Entry
Preferred Portal of Entry
• Many microorganisms can cause infections
only when they gain access through their
specific portal of entry.
• If they enter via another portal they cause
no response.
• Example: Flu is specific for respiratory
tissues and cannot cause disease if it enters
a different portal.
Virulence Can Be Expressed By the
Numbers of Invading Microbes
• LD50
- The number of microbes in a dose that
will kill 50% of inoculated test animals.
• ID50
- The dose
required to produce
a demonstratable
infection in 50% of
the test animals
Question?
• Agent A has a LD50 of 3.
• Agent B has a LD50 of 3000.
Which is the more virulent?
Remember: The Likelihood of Disease
Increases as the Number of Invading
Microbes Increases
Capsules Aid in Colonizing a Host
• Resist Phagocytosis
• Increase Adherence
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Streptococcus pneumoniae
Klebsiella pneumonia
Baillus anthracis
Yersinia pestis
Proteins in the Cell Wall Can Facilitate
Adherence or Prevent a Pathogen
From being Phagocytized
These Are Called M-Proteins
Bacteria Also Produce Enzymes To
Aid in Colonization
• Leukocidins / Destroy WBC
- Staphylococci and Streptococci
• Hemolysins / Lyse RBC
- C. perfringens, Staphylococci, Streptococci
• Fibrin Clot / Formed By Coagulase
- Staphylococci
• Kinases / Destroy Blood Clots
- S. pyogenes, S. aureus
Microbial Colonization Can
Result In Cell Damage and Death
• Direct Damage
Host cells can be destroyed via
pathogens multiplying and being released.
Production of Toxins by Microbes
• Toxins
Poisonous Substances
• Toxemia
Presence of Toxins in the Blood
• Toxigenicity
Ability to Produce Toxins
Exotoxins
• Exotoxins
Produced Inside the Bacteria as Part of their
Growth and Released into the Surrounding
Medium
• Streptococcal pyrogenic exotoxins A (SPEA) and B (SPE-B) have been implicated in
the pathogenesis of serious group A
streptococcal infections including
streptococcal toxic shock-syndrome.
• Clostridium difficile produces large oval
subterminal spores and two different toxins;
toxin A (an exotoxin causing fluid
accumulation in the intestine) and toxin B
(a cytopathic agent).
Endotoxins
• Endotoxins
A Structural Component in the Bacteria
That is Toxic and Released When the
Bacterial Cell Dies and in Lysed
• For example, the endotoxin of Bacillus
thuringiensis makes crystal-like inclusion
bodies next to the endospore inside the
bacteria. It is toxic to larvae of insects
feeding on plants, but is harmless to
humans.
Endotoxins Result In
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Fever
Chills
Weakness
Aches
Miscarriage
Shock
Death
Antitoxins
• Antibodies Produced Against Exotoxins
• An antitoxin is an antibody with the ability to
neutralize a specific toxin. Antitoxins are
produced by certain animals, plants, and
bacteria. Although they are most effective in
neutralizing toxins, they can kill bacteria and
other microorganisms.
• Cytotoxins
Work by Destroying Particular Parts of the Hosts Cell
or by Inhibiting Metabolic Functions (Ulcers)
• Neurotoxins
Inhibit or Prevent Nerve Transmission (Tetanus)
• Enterotoxins
Induce Fluid and Electrolyte Loss from Host Cells
(Cholera)
Viruses Avoid the Host’s
Immune Response by Growing
Inside Cells.
Visible Indications of Viral
Infections are Called Cytopathic
Effects
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Stopping Mitosis
Lysis
Formation of Inclusion Bodies
Cell Fusion
Chromosomal Changes
Cytocidal Effects (Cell Death)
Noncytocidal Effects (Cell
Damage but not Death)
Fungi, Protozoa, Helminthes, & Algae
• Symptoms Caused by
1. Capsules
2. Toxins
3. Allergic Reactions
• Resulting In
1. Damage to Host Tissue
2. Poisoning by Metabolic Wastes