Experiment 3 studied Koch's postulates for confirming plant pathogens. Koch's postulates require that (1) the suspected causal organism is constantly associated with the disease, (2) the organism can be isolated from an infected plant and grown in pure culture, (3) inoculation of a healthy host with the pure culture causes the original disease symptoms, and (4) the same pathogen can be re-isolated from experimentally infected plants. To test for the rice blast pathogen, the fungus would be isolated from infected rice, used to inoculate healthy rice, and observed for the same symptoms, then re-isolated to check it is the same fungus. Limitations of Koch's postulates include inability to culture some microbes and multiple organisms

1. Experiment 3
To study Koch’s postulates for the
confirmation of plant pathogens

2.  Robert Koch, a German scientist born in 1843,
is considered as Founder of Bacteriology.
 First to prove that bacteria caused disease.
 Formulated the Bacteriological techniques.
 One of Koch’s primary concerns was the
prevalent occurrence of anthrax among farm
animals in his area.
3

3.  Found the agent Bacillus anthracis by an
experimental process now known as Koch’s
postulates.
 He has converted his 4-room home into his
own medical laboratory and only with a
single microscope, Koch set to determining
the cause of this virulent disease.

4.  Koch perfected his methods of diagnostics and expanded on the
work of others.
 Koch invented the method of cultivating bacteria on nutrient
mediums, using potatoes as his source of nutrients for bacteria, and
created a medium that could be stored in dishes created by his
colleague Julius Richard Petri.

5. Potential pathogen
isolated from a samples
Symptoms Plant’s
condition
Diagnosis and effective treatment
of infection depends not just on
isolating an organism, but in
establishing a plausible link
between the laboratory findings,
recognised syndromes and the
infected plant’s condition

6. To determine Koch’s postulates:
1. the suspected causal organism must be constantly associated with the disease.
2. the suspected causal organism must be isolated from an infected plant and
grown in pure culture.
3. when a healthy susceptible host is inoculated with the pathogen from pure
culture and symptoms of the original disease must develop.
4. the same pathogen must be re-isolated from plants infected under experimental
conditions and shown to be identical to the organism originally isolated.
Koch's Postulates

7. Three rules for experimental proof of the pathogenicity of an
organism were presented in 1883 by a German bacteriologist,
Robert Koch; a fourth was appended by E. F. Smith in 1905.
These rules of proof are often referred to as Koch's Postulates.

8. Koch’s Postulates: Rice Blast
To determine which pathogen causes diamond shaped spot on rice you must:
1. describe the symptoms you see on the infected rice.
2. isolate the fungus in pure culture.
3. use the fungus that you isolated in pure culture to inoculate a healthy rice.
4. Record the symptoms that develop on the healthy rice following infection
with the cultured fungus. Are your observations the same as recorded
previously ?
5. Re-isolate the fungus and check that it is the same as observed initially.

9. Materials needed
1. Rice infected with Pyricularia oryzae
2. Slides and cover slips
3. Cotton blue stain
4. Compound microscope
5. Immersion oil
6. Sterile scalpels
7. Potato dextrose agar (PDA) plus antibiotic
8. Fresh rice seedlings

10. STEP-1

11. STEP-2

12. STEP-3

13. STEP-4

14. • Microorganisms that are unable to be cultured on
artificial media.
• Two or more organism work in synergy to cause a
disease.
• Symptoms and diseases can be causes by any one of
several microbes.