Host-microbe interactions

Question 1

Infection:

Answer 1

process whereby a microbe enters into a relationship with the host. It may or may not cause disease

Question 2

Infectious disease:

Answer 2

disease caused by an infection with a microbe. Some infections are communicable; others are not communicable and are not transmitted from patient to patient.

Question 3

Pathogenicity:

Answer 3

the ability (usually of a microbial species) to cause disease. Microbes that were able to cause disease readily in normal hosts were defined as frank pathogens (e.g., Yersinia pestis or Bacillus anthracis). Microbes that caused disease primarily in compromised hosts but less often in normal hosts were defined as opportunistic pathogens (e.g., Pseudomonas aeruginosa).
*** More contemporary idea: both microbial and host factors contribute to the outcome of an infectious disease. From this "damage-response" framework, a pathogen is a microbe capable of causing host damage

Question 4

Virulence:

Answer 4

the degree of pathogenicity (usually of a specific strain within a species). A highly virulent microbe was likely to cause disease when it was introduced into a host in small numbers
*** More contemporary idea: both microbial and host factors contribute to the outcome of an infectious disease. From this “damage-response” framework, virulence is the relative capacity of a microbe to cause damage in a susceptible host, and a virulence factor is a microbial component that can damage a susceptible host.

Question 5

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

Answer 5

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 6

Name the typical stages in pathogenesis of an infectious disease (6)

Answer 6

Encounter
Entry
Spread 
Multiplication 
Damage 
Outcome

Question 7

Describe and explain the importance of Encounter:

Answer 7

how the agent meets the host. Was the agent contracted endogenously (from the normal flora) or exogenously (from the environment)? Did the encounter occur by contact with other persons, animals, food, water, air, objects, etc.? What is the route of infection? What is the infectious dose?

Question 8

Describe and explain the importance of Entry:

Answer 8

how the agent enters the host. Must the microbe cross an epithelial barrier to cause disease? If so, does it enter the body actively (by invasion) or passively (by injection, bites, transfusion, organ transplantation, etc.)?

a) Colonization of body surfaces is the first step in pathogenesis. Many factors contribute to the extent and stability of colonization.
b) Adherence, mediated by specific binding of microbial surface components (such as pili or non-pilus adhesions) to receptors on host tissues = mechanism of microbial colonization.

Question 9

Describe and explain the importance of Spread:

Answer 9

how the agent spreads from the site of entry. Microbial products can promote or inhibit spread.

a) “spreading factors” facilitate the spread of microbes through tissues (i.e. Hyaluronidase, elastase, collagenase, etc.)
b) Coagulase inhibits spread of microbes by promoting the deposition of fibrin and helping to “wall off” and localize infections

Question 10

Describe and explain the importance of Multiplication:

Answer 10

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. The growth rate of microbes in the host (in vivo) may be much slower than their maximal growth rate in laboratory cultures (in vitro).

Question 11

Describe and explain the importance of Damage (and methods of damage):

Answer 11

How tissue damage is caused by the agent and/or the host response? Does the agent have specific virulence factors?

a) Direct damage to the host (e.g., microbial toxins and some bacterial enzymes)
b) Blocking or interfering with host defense mechanisms (e.g., antiphagocytic capsules, leukotoxins).

Question 12

Describe and explain the importance of Outcome:

Answer 12

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? The survival of obligate parasites depends on a satisfactory portal of exit from the infected host as well as on a portal of entry.

Question 13

Some factors (4) that differ among individuals and affect the microbiome include

Answer 13

1. Diet (breast-feeding, bottle feeding, solid food).
2. Suppression of microbial flora by treatment with antibiotics.
3. Anatomic abnormalities (e.g., blind loop syndrome).
4. Genetic differences between individuals (e.g., specific ABO and Se genotypes).

Question 14

Examples of physiologic importance of the microbiome includes

Answer 14

1. Effects on tissue/organ differentiation (normal vs. germfree animals).
2. Production of vitamins by gut flora.
3. Biochemical conversions (e.g., bilirubin degradation, drug metabolism, production of potential carcinogens, etc.)
4. Competition with pathogens for colonization of body surfaces. Source of agents for endogenously acquired infections.

Question 15

Compare several (4) disease paradigms that illustrate selected mechanisms of pathogenesis

Answer 15

A. Cholera—a toxin-mediated disease.
B. Pneumococcal pneumonia—acute inflammation caused by invasive, extracellular bacteria.
C. Tuberculosis—infection by a facultative intracellular bacterium.
D. Rheumatic fever—pathology mediated by an immune response