Host-microbe interactions

Question 1

Define and describe “infection”, “infectious disease”, “pathogenicity” and “virulence”.

Answer 1

Infection: The process whereby a microbe enters into a relationship with the host; may or may not cause disease.

Infectious disease: Disease caused by an infection with a microbe. Some infections are communicable, others are not communicable.

Pathogenicity: previously described as the ability of a microbe to cause disease, today it is considered that microbial and host factors contribute to the outcome of an infectious disease.

Virulence: The relative capacity of a microbe to cause damage in a susceptible host.

Question 2

Explain how a microbe is shown to be the cause of a specific disease.

Answer 2

Koch’s postulates:

1. Specific microbes are present regularly in characteristic lesions of the disease.
2. The specific microbes can be isolated and grown in vitro.
3. Injection of the cultured microbes into animals reproduces the disease seen in humans.
4. The specific microbes can be re-isolated from lesions of the disease in animals.

Question 3

Describe typical stages in pathogenesis of an infectious disease and explain their importance

Answer 3

1. Encounter: How the agent meets the host. (i.e. endogenous vs. exogenous; other people air, water, food; route of infection; infectious dose)
2. Entry: How the agent enters the host. (i.e. does it cross epithelial barrier; enter passively via injection, tranfusion, organ transplant or actively via invasion; what are the factors that contribute to colonization of body surfaces; what are the specific binding components that relate to adherance)
3. Spread: How the agent spreads from the site of entry
   - microbial products can promote spread (i.e. “spreading factors, e..g hyaluronidase, elastase, collagenase) or inhibit spread (e.g. coagulase promotes the deposition of fibrin, and helps wall off and localize infections)
4. Multiplication: how the agent multiplies in the host
   - Normal flora and pathogens must replicate in the host at rates that exceed their clearance by defense mechanisms.
5. Damage: how tissue damage is caused by the agent and/or the host response? Does the agent have specific virulence factors?
   - some microbial products cause direct damage to the host
   - some microbial products contribute to host damage by blocking/interfecting with host defense mechanisms
   - Most microbial products that cause direct damage to the host or interfere with host defense mechanisms are either surface components of microbes, or products that can be injected by the microbe directly into a target cell.
6. Outcome: does the microbial agent or the host win the battle, or do they learn to coexist?
   - what are the consequences of failure to eliminate the agent?

Question 4

Compare mechanisms of innate and acquired host defense against infections.

Answer 4

Innate: Complement-mediated lysis, opsonised phagocytosis and killing

Acquired: antibody production, T cells generate lymphokines

Question 5

Describe the composition and importance of the normal flora of the body

Answer 5

Skin: Staphylococcim corynebacteria, enteric bacilli
Oropharynx: a-hemolytic streptococci
Large intestine: enterococci
Vagina: Streptococci

Normal flora has effects of tissue/organ differentiation, produces vitamins in the gut, participates in biochemical conversions (e.g. bilirubin degradation, drug metabolism, production of certain carcinogens), and competes with pathogens for colonization of body surfaces; they are the source of agents for endogenously acquired infections.

Question 6

Compare several disease paradigms that illustrate selected mechanisms of pathogenesis

Answer 6

Cholera- toxin mediated disease
Pneumococcal pneumonia: acute inflammation caused by invasive extracellular bacteria
Tuberculosis: infection by a facultative intracellular bacterium
Rheumatic fever: pathology mediated by an immune response