Mechanisms of PathogenicityMechanisms of PathogenicityMicrobiology 2314

DefinitionsDefinitions

• 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 Portals of Entryof Entry

Preferred Portal of EntryPreferred 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 Virulence Can Be Expressed By the Numbers of Invading MicrobesNumbers 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?

Capsules Aid in Colonizing a HostCapsules Aid in Colonizing a Host

• Resist Phagocytosis• Increase Adherence

– Streptococcus pneumoniae– Klebsiella pneumonia– Baillus anthracis– Yersinia pestis

Remember: The Likelihood of Disease Increases as the Number of Invading

Microbes Increases

Proteins in the Cell Wall Can Facilitate Proteins in the Cell Wall Can Facilitate Adherence or Prevent a Pathogen Adherence or Prevent a Pathogen

From being Phagocytized From being Phagocytized These Are Called M-ProteinsThese Are Called M-Proteins

Bacteria Also Produce Enzymes To Bacteria Also Produce Enzymes To Aid in ColonizationAid 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 Microbial Colonization Can Result In Cell Damage and DeathResult In Cell Damage and Death

• Direct Damage

Host cells can be destroyed via

pathogens multiplying and being released.

Production of Toxins by MicrobesProduction of Toxins by Microbes• Toxins

Poisonous Substances

• ToxemiaPresence of Toxins in the Blood

• ToxigenicityAbility to Produce Toxins

ExotoxinsExotoxins• Exotoxins

Produced Inside the Bacteria as Part of their Growth and Released into the Surrounding Medium

• Streptococcal pyrogenic exotoxins A (SPE-A) 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).

• EndotoxinsA 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.

EndotoxinsEndotoxins

Endotoxins Result InEndotoxins Result In

• Fever

• Chills

• Weakness

• Aches

• Miscarriage

• Shock

• Death

AntitoxinsAntitoxins

• 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.

• CytotoxinsWork by Destroying Particular Parts of the Hosts Cell or by Inhibiting Metabolic Functions (Ulcers)

• NeurotoxinsInhibit or Prevent Nerve Transmission (Tetanus)

• EnterotoxinsInduce Fluid and Electrolyte Loss from Host Cells (Cholera)

Viruses Avoid the Host’s Immune Response by Growing

Inside Cells.

Visible Indications of Viral Visible Indications of Viral Infections are Called Cytopathic Infections are Called Cytopathic

EffectsEffects• 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, & AlgaeFungi, 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