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Kochs postulates in the 21st century

Discussion in 'XMRV Research and Replication Studies' started by natasa778, Feb 1, 2010.

  1. natasa778

    natasa778 Senior Member

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    http://www.virology.ws/2010/01/22/kochs-postulates-in-the-21st-century/

    For thousands of years, epidemics of contagious diseases were believed to be caused by the wrath of the gods, configuration of stars, or miasma. The association of specific microorganisms with disease came about as a consequence of the work of the German physician Robert Koch. He formulated a set of criteria that could be used to identify the pathogen responsible for a specific disease. These criteria came to be known as Kochs postulates:

    1. The organism must be regularly associated with the disease and its characteristic lesions.
    2. The organism must be isolated from the diseased host and grown in culture.
    3. The disease must be reproduced when a pure culture of the organism is introduced into a healthy, susceptible host.
    4. The same organism must be reisolated from the experimentally infected host.

    Koch applied these criteria to show that anthrax, a common disease of cattle, was caused by the bacterium Bacillus anthracis, and that tuberculosis in humans was caused by a different bacterial species. His postulates provided a framework for proving the role of microbes in disease. As a consequence of his work, the study of infectious disease was placed on a secure scientific foundation, which ultimately made possible rational treatment and control.

    Despite the importance of Kochs postulates in the development of microbiology, they have severe limitations, which even Koch realized. For example, he believed that cholera and leprosy were caused by microbes, but could not fulfill all four postulates. Furthermore, Koch knew that the putative agent of cholera, Vibrio cholerae, could be isolated from both sick and healthy people, invalidating postulate #2.

    The limitations of Kochs criteria are even more obvious when we consider viral diseases, which were not yet discovered when the postulates were formulated. Thomas Rivers, who has been called the father of modern virology, wrote:

    It is unfortunate that so many workers blindly followed the rules, because Koch himself quickly realized that in certain instances all the conditions could not be met. . . . Thus, in regard to certain diseases, particularly those caused by viruses, the blind adherence to Kochs postulates may act as a hindrance instead of an aid.

    Many viruses do not cause illness in all infected individuals, a requirement of postulate #1. An example is poliovirus, which causes paralytic disease in about 1% of those infected. Further compromising postulate #1 is the fact that infection with the same virus may lead to markedly different diseases, while different viruses may cause the same disease. Postulates #2 and #3 cannot be fulfilled for viruses that do not replicate in cell culture, or for which a suitable animal model has not been identified.

    The application of nucleic acid-based methods of microbial identification has made Kochs postulates even less applicable. Polymerase chain reaction and high-throughput sequence analysis have revealed a great deal about microbes that are associated with pathology or disease, but proving causation has become even more difficult as the number of uncultivable viruses rapidly multiplies. Nucleic acid based detection methods are so sensitive that they detect small numbers of viruses that may occur in the absence of disease. The use of these new methods have lead to revised versions of Kochs postulates that are fundamentally sound: both hepatitis C virus and human papillomaviruses were convincingly shown to be causative agents of hepatitis and cervical cancer, respectively, long before methods were developed for propagation of the viruses in cell culture.

    Here are Kochs postulates for the 21st century as suggested by Fredricks and Relman:

    1. A nucleic acid sequence belonging to a putative pathogen should be present in most cases of an infectious disease. Microbial nucleic acids should be found preferentially in those organs or gross anatomic sites known to be diseased, and not in those organs that lack pathology.
    2. Fewer, or no, copy numbers of pathogen-associated nucleic acid sequences should occur in hosts or tissues without disease.
    3. With resolution of disease, the copy number of pathogen-associated nucleic acid sequences should decrease or become undetectable. With clinical relapse, the opposite should occur.
    4. When sequence detection predates disease, or sequence copy number correlates with severity of disease or pathology, the sequence-disease association is more likely to be a causal relationship.
    5. The nature of the microorganism inferred from the available sequence should be consistent with the known biological characteristics of that group of organisms.
    6. Tissue-sequence correlates should be sought at the cellular level: efforts should be made to demonstrate specific in situ hybridization of microbial sequence to areas of tissue pathology and to visible microorganisms or to areas where microorganisms are presumed to be located.
    7. These sequence-based forms of evidence for microbial causation should be reproducible.

    Fredericks DN, & Relman DA (1996). Sequence-based identification of microbial pathogens: a reconsideration of Kochs 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

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    ... imo until someone carries out, and later someone replicates, studies on TISSUE samples we won't be any wiser as to whether XMRV is pathogenic in CFS, autism etc....
  2. Hi Natasa,

    Thank you so much for posting this... helped me to understand a lot about the science behind XMRV studies.
    I thought the most recent work WPI had done was showing XMRV was infectious from and to tissue samples... is that not the case? Was it just blood plasma?

    Rachel xx
  3. natasa778

    natasa778 Senior Member

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    Judy said in her talk that they are suspecting tissue reservoirs of xmrv - they are not sure which tissue but they think it is from there that it 'leaks' to blood cells....

    my first bet would be endothelial tissue - gut lining, brain lining (blood vessel lining), microglia, and autonomic nervous system maybe... Just thinking out loud. For example in autism the gut lining is very similar to what is found in HIV positive, not sure if that the case in CFS but XMRV would fit the picture perfectly.
  4. Advocate

    Advocate Senior Member

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    I wasn't sure what "copy number" meant. Here's a nice little 30 second talk on copy numbers, by Francis Collins at the National Human Research Institute.
  5. kurt

    kurt Senior Member

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    This is interesting, a PCR version of Koch.

    Note that even from the original set of Koch's Postulates, a culture study may confirm that a pathogen can infect host tissues, but to test an actual causal model (which has not even been attempted yet with XMRV for CFS) the culture study would have to produce the CFS disease in an animal model (don't really want to do that to a rat or a chimp, but that type of confirmation would be required to really prove XMRV causes CFS). Also, treatment and proof of eradication of the pathogen would have to be shown to lead to cessation of the disease in a human host.

    Anyway, neither set of these postulates addresses the modern science of 'Systems Biology', for that a new set of criteria is needed. One that addresses interactions between multiple complex mechanisms of disease.

    Here is an interesting article that suggests something like that:

    http://www.pnas.org/content/106/1/6.full

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