• Google Plus
  • Facebook
  • Twitter
  • Linkedin
  • Pakistan Agriculture and Farming Cover

    Welcome to PakAgriFarming

    PakAgriFarming is commited to high standards in extension of agriculture in Pakistan through simple and factual information and topics related to the hottest issues in Pakistan Agriculture.

  • Approved cotton varieties for Punjab 2014

    کپاس کی کاشت کے لیے موزوں اقسام

    پنجاب میں کپاس کی کاشت کا آغاز ہو نے والا ہے اور پیشتر علاقوں میں میں رنیع کی فصل کی برداشت کا وقت قرہب ہے۔ اس لیے بروقت کپاس کے بیج کا انتخاب اچھی پیداوار حاصل کرنے کی طرف پہلا قدم ہے۔۔۔

  • Introduction, characteristics and objectives of tillage

    Intoduction and Objectives of Tillage

    Tillage is the primary requirement before any cropping activity. Perhaps, if one is ignorant about tillage, he knows nothing about farming. So, let's look into a brief intro...

  • The Next 10 Years of Tunnel Farming in Pakistan

    The Next 10 Years of Tunnel Farming in Pakistan

    It's been 10 years since the introduction of tunnel farming in Pakistan, the conversion rate has been slow but are there any chance of rapid growth of tunnel farming in Pakistan? Let's disover...

  • Wheat Production Technology for Rainfed Areas of Punjab for 2013-14

    Wheat Production Technology for Rainfed Areas of Punjab for 2013-14

    Wheat is the most important cereal crop in Pakistan and staple food. Wheat has a large share of cultivated land in rabi season and hence large production is expected...


Tuesday, January 15, 2013

Koch's Postulates and their present Modifications

Posted by Shakil Shaukat On Tuesday, January 15, 2013

In 1884, Robert Koch and Friedrich Loeffler tried to define criteria for identification of any pathogen associated with a particular disease in living organisms. These postulates were further refined by Koch and published in 1890. Koch, primarily set these postulates to establish the association of Bacillus anthracis with the anthrax of cattle. And he also established that tuberculosis in human is caused by a different species of bacteria. 
Koch's postulate is being followed for leaf spots of rice (Oryza sativa)

Koch’s postulates

1. The organism must be found on all the diseased host tissue but not on the healthy host. 
2. The organism can be isolated from the host and purified on artificial culture medium. 
3. The organism from pure culture, when inoculated on the susceptible host must reproduce the symptoms as in step 1. 
4. The organism can be re-isolated from the artificially inoculated host on pure culture medium. 


Koch’s postulates, although have significant role in the development of microbiology but still have their limitation and Koch himself was aware of this. For example, he believed that leprosy and cholera were caused by microbes but could not fulfill all postulates. In case of cholera, Vibrio cholerae, could be isolated from both sick and healthy people, hence a negation of postulate #1.
The limitations of Koch’s postulated were more prominent in case of viral diseases which were not discovered at the time these postulates were formulated.
Most of the viruses which are present in living organisms do not cause disease which is negation of postulate #1. For example, a poliovirus which is present in most of the individual causes paralytic disease in only less than 1% of infected individuals. Similarly most of the viruses cannot be isolated on pure medium or for which not medium has been identified (Negation of postulate #2 and #3). Thomas Rivers, “Father of modern virology” also commented that in some instances when the conditions are not met for Koch’s postulated, then Koch’s postulated become hindrance to work instead of assistance.

Present Horizons

The application of nucleic acid based methods of microbial identificaion, polymerase chain reaction (PCR) and high throughput analysis have revealed a great deal about the association of highly uncultivable viruses (and which multiplies rapidly) and they are found associated with tissues even without the development of the diseases. The use of these modern methods have sought for a modification of the Koch’s postulates which are otherwise sound.

Modifications of Koch's Postulates

Fredricks and Relman (1998) modified Koch’s postulates for viruses and other nucleic acids (DNA/RNA) based pathogens.
1. A nucleic acid sequence (DNA/RNA) of known pathogen should be found associated in most of the cases with a disease which can found in those organs or gross anatomical sites which are having symptoms of the disease.
2. Pathogen-associated nucleic acid sequences may or may not be present in fewer numbers from the host or tissues without disease.
3. The copy number (count) of the pathogen-associated nucleic acid sequences should decrease or become undetectable in case of resolution of disease. With clinical recurrence, the case will be vice versa.
4. In case of association of the nucleic acid sequences with the tissue before disease development or when sequence copy number correlates severity of disease or pathology; then the sequence-disease relationship is more likely to be causal relationship.
5. The nature of the microorganism inferred from the available sequence should be consistent with the known biological characteristics of that group.
6. The relationship between the sequence and tissue should be tried to establish at the cellular level; efforts should be made to demonstrate the hybridization of the microbial sequence to the areas of tissue pathology and to visible microorganisms or to areas where microorganism are thought to be involved. These sequence based forms of evidences for microbial causation should be reproducible.


Fredericks DN, & Relman DA (1996). Sequence-based identification of microbial pathogens: a reconsideration of Koch’s postulates. Clinical microbiology reviews, 9 (1), 18-33 PMID: 8665474
Lipkin, W. (2008). Microbe hunting in the 21st century Proceedings of the National Academy of Sciences, 106 (1), 6-7 DOI: 10.1073/pnas.0811420106