What are the 4 Koch's
postulates?
Koch's postulates are a set of criteria used to establish a causal
relationship between a microorganism and a disease.
Postulate
1. The microorganism must be present in every case of the disease but absent
from healthy organisms.
Koch's Experiment:
- Koch developed a staining technique to examine human tissue.
- Mycobacterium tuberculosis cells could be identified in diseased tissue.
Exceptions 1: In the case of some diseases, microorganism may be present even without
showing any signs or symptoms of disease. Polio, AIDS and Cholera can be
carried asymptomatically, which violates the first postulate.
Postulate 2. The microorganism must be isolated from the diseased host and grown in pure culture.
Koch's Experiment:
Koch grew M. tuberculosis in pure culture on coagulated blood
serum.
Exceptions 2: Some microorganism cannot be
cultured in an artificial medium
Example: Treponema pallidum (Syphilis), Mycobacterium leprae (leprosy)
Rickettsia, chlamydia multiply only within cell
Exceptions 3: Subcellular disease-causing
particle cannot be cultured in an artificial medium
Viruses, prions etc
Exceptions 4: A diseases
may be caused by many causative microorganism
Diarrhea and Meningitis
are caused by bacterial, viral, fungal, and parasitic pathogens.
Exception 5: Subcellular disease-causing particle cannot be cultured in an artificial
medium such as viruses, prions etc.
Exception 6: Some microorganism can cause many diseases
Streptococcus pyogenes can cause sore throat but also cause scarlet fever and bone
inflammation. Mycobacterium tuberculosis can cause disease symptoms
in lungs, bone and even skin
Postulate 3. The same disease must result when the
isolated microorganism is inoculated into a healthy host.
Koch's Experiment:
- Koch injected cells from the pure culture of M.tuberculosis into guinea pigs.
- The guinea pigs subsequently died of tuberculosis
Exception 7: Koch himself proved in regard to both
tuberculosis and cholera, not all organisms exposed to an infectious agent will
acquire the infection
Exception 8: For human limited diseases like AIDS caused by HIV virus, without a
suitable nonhuman host, a researcher cannot evaluate postulate 3 as
deliberately infecting humans presents obvious ethical concerns.
Postulate 4. The
same microorganism must be isolated again from the diseased host.
Koch's Experiment:
- Koch isolated M. tuberculosis from the dead guinea pigs and was able to again culture the microbe in pure culture on coagulated blood serum.
Exception 9: Many infections by microbes, considered the underlying cause of a
disease, are absent from the lesions that ultimately develop.
Group A streptococcus infection is considered to be the underlying cause
of rheumatic fever. The infection is long gone prior to the appearance of the
valvular and endocardial lesions of rheumatic fever.
Human papillomavirus (HPV) is considered to be the underlying cause of nearly all cases of squamous carcinoma of the uterine cervix. After many years; following the early papillomavirus infections, may lack recoverable virus.
Difference between Koch's Postulates and Molecular Koch's postulate
A great way to thank and support us.
- Visit our TPT store for Koch's Postulate Worksheet
- Download free resources or
purchase some.
- Please rate the product and
follow us on store.
Thank you so much
