Lecture 16_250 review

Exam 2 Review Topics

Lecture 16: Microbial Pathogenesis

• Key concepts related to how microorganisms cause disease.

Koch’s Postulates

• Steps of Koch’s Postulates

  1. The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.

  2. The microorganism must be isolated from a diseased organism and grown in pure culture.

  3. The cultured microorganism should cause disease when introduced into a healthy organism.

  4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

• Limitations of Koch's Postulates

  • Not all pathogens cause disease in every individual.

  • Some microorganisms cannot be cultured in a lab setting.

  • Ethical concerns with using humans in experimentation.

  • Complexity in diseases caused by multiple pathogens or co-infections.

Pathogenicity

• Definition: The ability of an organism to cause disease.

• Infectivity: Ability of a pathogen to establish an infection.

• Virulence: Degree of pathogenicity; how severe the disease is.

• ID50 (Infectious Dose): Number of pathogens required to infect 50% of hosts.

• LD50 (Lethal Dose): Number of pathogens required to kill 50% of hosts.

• Higher values of ID50 indicate lower infectivity; lower values indicate higher virulence.

Transmission of Pathogens

• Direct Transmission: Person-to-person contact (e.g., touching, kissing).

• Indirect Transmission: Spread through an intermediate object (e.g., contaminated surfaces).

• Vehicle Transmission: Spread through a common source like food or water.

• Vector Transmission: Spread via a living organism, usually an arthropod (e.g., insects).

Stages of Pathogenesis

• Primary Pathogen: Causes disease in a healthy host.

• Opportunistic Pathogen: Causes disease when the host's defenses are compromised.

• Exposure: Initial contact with the pathogen.

• Portals of Entry: Routes through which pathogens enter the body (e.g., skin, mucous membranes).

• Adhesion: Ability of pathogens to attach to host tissues.

  • Importance: Essential for colonization and infection.

  • Examples of Adherence Factors: Fimbriae, capsules, specialized adhesion molecules.

• Invasion: Penetration and spread of pathogens within the host tissues.

  • Virulence Factors: Traits that enhance a pathogen's ability to cause disease; often found on pathogenicity islands.

Extracellular Factors

• Exoenzyme: Enzymes secreted by pathogens to aid in invasion and obtain nutrients.

• Toxins: Substances that contribute to disease severity.

  • Endotoxins: Components of bacterial cell walls (e.g., lipopolysaccharides) that cause systemic symptoms.

  • Exotoxins: Secreted proteins that cause damage to host cells. Known for their specificity and potency.

Intracellular Factors

• Obligate Intracellular Pathogens: Must live within a host cell to replicate.

• Facultative Intracellular Pathogens: Can live inside or outside host cells.

• Role of Secreted Effector Proteins: Modulate host cell functions during invasion and replication.

• Step-by-Step of Bacterial Host Invasion: Involves adherence, invasion, evasion of host defenses, and replication.

Infection Defined

• Infection: The invasion and multiplication of pathogens within the host.

• Local Infection: Confined to a specific area of the body.

• Focal Infection: Starts in one location and spreads to others.

• Systemic Infection: Involves the entire body, often via the bloodstream.

• Primary Infection: Initial infection by a pathogen.

• Secondary Infection: Occurs after primary infection, typically due to compromised host defenses.

Course of Infection

• Clinical Signs vs. Clinical Symptoms: Signs are objective findings (e.g., fever, rash); symptoms are subjective feelings (e.g., pain, fatigue).

• Periods of Infection:

  • Incubation Period: Time from exposure to the onset of symptoms.

  • Prodromal Period: Early signs and symptoms appear but are not specific.

  • Illness Period: Symptoms are most severe; diagnostic criteria met.

  • Decline Period: Symptoms begin to subside.

  • Convalescence Period: Recovery phase; body repairs itself.