germ theory of disease

1. 1.2.2. The germ theory of disease (Koch's
Postulates)
 The germ theory of disease states that many diseases are caused by
microorganisms such as bacteria, viruses, protozoa, or fungi.
 These diseases are caused by the growth and replication of microorganisms.
 The germ theory of disease was devised by Louis Pasteur.
 He also performed various experiments to demonstrate the relationship
between microorganisms and diseases.
 Building on the early work of Louis Pasteur and the germ theory of disease, Robert
Koch established the basic scientific requirements used to demonstrate that each
specific disease is caused by a specific microorganism.
 These requirements were based on Koch’s experiments with anthrax isolated from
diseased hosts, and are known as “Koch’s Postulates”. Koch’s Postulates consist of the
following four rules:

2. 1. A specific disease is caused by a specific organism. A specific
organism can always be found in association with a given disease
2. The organism should be isolated and grown in the lab into a pure
culture.
3. When the artificially cultured organism inoculated into health but
susceptible animal it should produce symptoms of the same
diseases
4. The organism should be re-isolated from artificially infected
animal and grown into pure culture in the lab.

3. There are four Kotch’s postulate on diseased plant
 To demonstrate Koch’s Postulates, you must do the following:
 (i) Describe and record the symptoms shown.
 (ii) Isolate the suspected pathogen from the infected plant
material and establish a pure culture.
 (iii) Inoculation of new healthy plant material.
 (iv) Re-describe and record the symptoms shown by the new
plant. Check that these are the same as your original observations.
 (v) Re-isolate the organism. Check that this is the same as that
isolated previously.

4. Drawbacks (Limitations) of Kotch’s postulate
1. Not every microorganism associated with diseases. ex. Normal flora do not cause
diseases
2. Many health people carry pathogens but do not exhibit symptoms of the
disease (carriers)
3. Certain disease develops only when an opportunistic pathogen invades a weakened
host. Opportunistic microorganisms can cause diseases only in immune compromised
individuals.
4. Not all diseases are caused by microorganisms.
Ex. Diabetes, Asthma, hypertension etc.
5. Some microbes are very difficult or impossible to grow in the laboratory in artificial
media, such as most viruses and some bacteria
ex. Treponema pallidivm and M.leprae are uncultivable organisms

5. 1. Emphasizes the importance of lab cultivation of
microorganism in artificial growth media
2. Showed that a specific microbe has specific activity
3. Contributed to the development of pure culture