Koch: Limitations of his postulates.
Questions that need answers.
Can Koch’s Postulates be applied to all microorganisms that cause disease?
Due to numerous limitations of the postulates they cannot be applied to all microorganisms. According to Koch for a microorganism to be considered as the cause of disease it must have met a series of conditions which he called Koch’s Postulates. However, he realised that it is not always when these conditions were met, this gives the bases of this answer to refute that his postulates can be applied to all microorganisms that can cause disease.
Some particular examples of microorganisms that deviate from his postulates include Vibrio cholerae and Salmonella enterica in the case of asymptomatic carriers of cholera and typhoid fever.
Mycobacterium leprae (M. leprae). This is a slow-growing bacteria that causes leprosy. It cannot be grown in pure culture; hence it will not be compliant with Koch’s second postulate which requires the organism to be grown in pure culture.
In addition, not all organisms exposed to an infectious agent will acquire the infection as in the cases of tuberculosis and cholera (3rd Postulate). Non-infection may be due to such factors as general health and proper immune functioning; acquired immunity from previous exposure or vaccination; or genetic immunity, as with the resistance to malaria conferred by possessing at least one sickle cell allele Robinson, C. (1958).
Not all diseases have bacterial etiologies that are genetic, degenerative or congenital. The bacterium Mycobacterium tuberculosis does not only cause lung disease but can also cause diseases in the skin, bones, and internal organs. Moreover, the coccus bacterium Streptococcus pyogenes does not only cause sore throat but can also cause scarlet fever, erysipelas, osteomyelitis (bone inflammation), among other diseases. If diagnosticians use clinical signs and symptoms together with laboratory procedures, the mentioned bacterial infections can usually be distinguished from infections to similar organs by other pathogenic microbes (Alcamo 2002; Ingraham 2000)
Generally harmless bacteria that exist in an organism may be the causative agent of disease if
It has acquired extra virulence factors making it pathogenic.
It gains access to deep tissues where it is not supposed to be, this may occur via trauma and surgery among other ways.
It infects an immunocompromised patient (an individual with impaired immune system).
Not all people infected by a bacteria may develop disease-subclinical infection is usually more common than clinically obvious infection.
How can you apply Koch’s postulates to viruses and viral diseases?
Koch’s postulates cannot be applied to viruses because, a number of viruses do not cause illness in all infected individuals, as required by first postulate. Poliovirus, which causes paralytic disease in about 1% of those infected is the causative agent of poliomyelitis (commonly known as polio) and is a human enterovirus. Further compromising the first postulate is 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. Postulate number 4 is also difficult to undertake because it would be unethical to expose people with the viruses (Fredericks, 1996). Furthermore, ethical considerations prevent researchers from fulfilling the third postulate in serious diseases affecting only human hosts. Additional complications arise from situations in which the same organism causes different diseases under different circumstances or diseases that may be caused by a community of microorganisms rather than a single pathogen. In various situations, a human host shows certain signs and symptoms that are associated only with a certain microbe and its disease (Wheelis 2007). For instance, the bacteria responsible for tetanus and diphtheria cause distinctive signs and symptoms that no other microbe can produce. They are unequivocally the only bacteria that produce their respective diseases. However, there are cases wherein different microbes show similar signs and symptoms that you cannot identify what specific microbe causes the disease. For instance, the inflammation of the kidney (nephritis) can involve the action of several different pathogens, all of which cause the same signs and symptoms. Hence, it is often hard to know which particular microbe is causing a disease (Alcamo 2002). Koch believed that the signs and symptoms of anthrax are unique for "anthrax" but actually not.
How can one apply Koch’s postulates to non-cultivatable microorganisms?
Koch has postulated that the pathogen isolated from the blood of a diseased animal should be cultivated in an artificial culture medium like chicken or beef broth. Microbiologists after Koch found out that there are infectious microorganisms that cannot multiply in artificial media but can only multiply on living cells. This propelled microbiologist to modify Koch’s postulate and to find alternative means of culturing microorganisms (Alcamo 2002). For example, the failure of microbiologists to isolate the bacterium that causes Legionellosis necessitated them to get a lung tissue sample from an infected person and inject it into guinea pigs. They also got lung tissue from a healthy human and injected it into other guinea pigs. They found out that the guinea pigs inoculated with lung tissue from the infected human developed pneumonia (major symptom of legionellosis) while the others injected with lung tissue from the unafflicted person did not develop pneumonia. Tissue samples from the diseased guinea pigs were cultured in the yolk sacs of chick embryos because the microbes cannot grow in artificial media (e.g. chicken and beef broths). Extremely small microbes were found at the embryos and through the aid of an electron microscope, microbiologists saw rod-shaped bacteria (Ingraham 2000)
References
Koch’s postulates in the 21st century. (2016). Virology.ws. Retrieved 24 September 2016, from http://www.virology.ws/2010/01/22/kochs-postulates-in-the-21st-century/
Robinson, C. (1958). Koch's Postulates and the Modern Era in Virus Research.Canadian Medical Association Journal,79(5), 387. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1830405/?pageindex=1
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
The Exceptions to Robert Koch’s Postulates. (2016). Health Guide Info. Retrieved 26 September 2016, from http://www.healthguideinfo.com/infectious-disease/p21175/
Alcamo, Edward. 2002. Microbes and Society: An Introduction to Microbiology. Jones & Bartlett Publishers.
Ingraham, John. 2000. Introduction to Microbiology. Pacific Grove, Calif.: Brooks/Cole Pub.
Wheelis, Mark. 2007. Principles of Modern Microbiology. Jones & Bartlett Publishers.
