MicrobiologyBytes: Microbiology Notes: Koch's Postulates Updated: October 19, 2004 Search

Koch's Postulates

Establishing a specific microbe as the cause of an infective disease:

The Koch - Henle postulates:

  1. Isolate the organism from every case
  2. Propagate in pure culture in vitro
  3. Reproduce disease by injecting the organism into a suitable recipient
  4. Re-isolate the organism

OK for major acute diseases like plague, small pox, typhoid.

But what about:

Pathogens cause infection and the establishment and outcome of infection is DOSE-RELATED:

ID50 (dose required to produce specified outcome in 50% of target population) is shifted :

Any specific attribute of an organism whose loss decreases virulence is called:

a virulence factor or virulence determinant.


Formation of microbe human associations:

Encounter may be Exogenous or Endogenous.

Factors affecting progress of a pathogen:

Virulence factors can be related to the stages of infection, e.g:

1. Entry / Establishment:

2. Spread:

3. Multiplication:

4. Damage:

It makes sense that these factors could all contribute to virulence.

But proof of involvement is important:

Hence: "Molecular Koch's Postulates":

  1. Identify gene (or gene product) responsible for virulence determinant
  2. Show gene present in strains of bacteria that cause the disease
  3. Not present in avirulent strains
  4. Disrupting the gene reduces virulence
  5. Introduction of cloned gene into avirulent strain confers virulence.
  6. The gene is expressed in vivo
  7. Specific immune response to gene protects

The "mug shot " book (3 groups):

1. Distinctive pathogens: Organisms for which the Koch Henle postulates have been fulfilled.
1 microbe ->1 disease, e.g. the causal agents of plague, anthrax, diphtheria, cholera, typhoid etc.

2. Agents of sepsis: Organisms regularly isolated from patients with symptomatic infections.
Many microbes ->1 disease, e.g. agents causing pneumonia, wound infections, urinary tract infections, septicaemia

3. Pluripotential pathogens: 1 microbe ->many diseases, e.g. S. aureus, E. coli.


Isolation of a pathogen (particularly in groups 2 or 3) does not necessarily mean that a patient has any of the diseases associated with that organism:

Opportunistic pathogens:
Staphylococcus aureus
Staphylococcus epidermidis
Bacillus subtilis
Haemophilus influenzae
capsule type b+
Haemophilus influenzae
capsule type b-
E. coli
E. coli
E. coli

List three sources of evidence that could be used to indicate that a particular organism causes a particular disease if there is no suitable laboratory model for the disease.



1) John Hunter showed that infections could be transmitted by sex:
True False

2) Robert Koch was a German GP:
True False

3) A pathogen is an organism that is never found on a healthy person:
True False

4) The human body is normally host to around:
105 1014 1023    bacterial, fungal and protozoan cells

5) Infection does not always lead to symptoms:
True False

6) Some organisms can be pathogens and commensals:
True False

7) An opportunistic pathogen has a relatively low infectious dose:
True False

8) Damage in infection nearly all results from the production of microbial toxins:
True False

9) A particular strain of an organism can be both a pathogen and a non pathogen:
True False

10) Endogenous infections are caused by members of the patient’s normal flora:
True False

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