Introduction to Pathogenicity

Question 1

Learning Objectives

Answer 1

1. The germ theory of infectious diseases and Koch’s postulates
2. The difference between pathogenicity, virulence and infectivity
3. The prevalence of communicable diseases as causes of worldwide mortality
4. The definition of infectious dose (ID)

Question 2

What are the original Koch’s postulates?

Answer 2

1. microbes must be associated with symptoms of the disease and must be present at the site of the infection
2. microbes must be isolated from the lesions of disease and able to be grown as a pure culture
3. a pure culture of the microbe, when inoculated in a susceptible host must reproduce the same disease
4. the microbe must be re-isolated in pure culture from the experimentally infected host

Question 3

Koch’s postulates (1884 & 1890)

Answer 3

Guidelines to establish a relationship between a microbe and a disease

Question 4

What are the limitations of Koch’s postulates?

Answer 4

• only applicable when the pathogen can be cultured
• some microbes are co-dependent with other species
• some diseases are polymicrobial

Question 5

Germ Theory of Infectious disease

Answer 5

Louise Pasteur and Friedrich Gustav Jacob Henle (1800s)
states that microorganisms known as “pathogens” or “germs” can cause disease

Question 6

Pathogen

Answer 6

an organism that cause disease and damange the host

many are opportunistic

Question 7

Co-evolution with microbes

Answer 7

• evolving life form has been surrounded by bacteria
• co-evolution of plants and animals with bacteria and many types of ecological interactions between microbe and host

i.e. mutualism, cooperation, commensalism, pathogenicity, competition

Question 8

Pathogenicity

Answer 8

refers to the ability of an organism to cause disease and damage the host

Question 9

Virulence

Answer 9

the degree of pathology caused by the organism

the severity of the disease

Question 10

Infectivity

Answer 10

pathogen’s capacity for horizontal transmission and how frequently it spreads among hosts

Question 11

Infectious Dose (ID50)

Answer 11

dose of bacteria at which 50% of the hosts are infected

lower ID50 = increase in mortality

Question 12

Mortality (Case Fatality)

Answer 12

the percentage of the infected hosts that die

increase in mortality = lower ID50