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Koch also assumed that all pathogens are microorganisms that can be grown in pure culture (postulate 2) and that animals could serve as reliable models for human disease. However, we now know that not all pathogens can be grown in pure culture, and many human diseases cannot be reliably replicated in animal hosts. Viruses and certain bacteria, including Rickettsia and Chlamydia , are obligate intracellular pathogens that can grow only when inside a host cell. If a microbe cannot be cultured, a researcher cannot move past postulate 2. Likewise, without a suitable nonhuman host, a researcher cannot evaluate postulate 2 without deliberately infecting humans, which presents obvious ethical concerns. AIDS is an example of such a disease because the human immunodeficiency virus (HIV) only causes disease in humans.

  • Briefly summarize the limitations of Koch’s postulates.

Molecular koch’s postulates

In 1988, Stanley Falkow (1934–) proposed a revised form of Koch’s postulates known as molecular Koch’s postulates . These are listed in the left column of [link] . The premise for molecular Koch’s postulates is not in the ability to isolate a particular pathogen but rather to identify a gene that may cause the organism to be pathogenic.

Falkow’s modifications to Koch’s original postulates explain not only infections caused by intracellular pathogens but also the existence of pathogenic strains of organisms that are usually nonpathogenic. For example, the predominant form of the bacterium Escherichia coli is a member of the normal microbiota of the human intestine and is generally considered harmless. However, there are pathogenic strains of E. coli such as enterotoxigenic E. coli ( ETEC ) and enterohemorrhagic E. coli (O157:H7) (EHEC). We now know ETEC and EHEC exist because of the acquisition of new genes by the once-harmless E. coli , which, in the form of these pathogenic strains, is now capable of producing toxins and causing illness. The pathogenic forms resulted from minor genetic changes. The right-side column of [link] illustrates how molecular Koch’s postulates can be applied to identify EHEC as a pathogenic bacterium.

Molecular Koch’s Postulates Applied to EHEC
Molecular Koch’s Postulates Application to EHEC
(1) The phenotype (sign or symptom of disease) should be associated only with pathogenic strains of a species. EHEC causes intestinal inflammation and diarrhea, whereas nonpathogenic strains of E. coli do not.
(2) Inactivation of the suspected gene(s) associated with pathogenicity should result in a measurable loss of pathogenicity. One of the genes in EHEC encodes for Shiga toxin, a bacterial toxin (poison) that inhibits protein synthesis. Inactivating this gene reduces the bacteria’s ability to cause disease.
(3) Reversion of the inactive gene should restore the disease phenotype. By adding the gene that encodes the toxin back into the genome (e.g., with a phage or plasmid), EHEC’s ability to cause disease is restored.

As with Koch’s original postulates, the molecular Koch’s postulates have limitations. For example, genetic manipulation of some pathogens is not possible using current methods of molecular genetics. In a similar vein, some diseases do not have suitable animal models, which limits the utility of both the original and molecular postulates.

Questions & Answers

Differentiation between electron, proton and neutron
Zainab Reply
proton .possitive charge electron . negative charge neutron . having no charge
what are the roles of microorganisms in human being
Buhari Reply
some causes disease, others are not disease causing
they're necessary in our digestive system+the skin,everywhere actually the number of them in the human body alone is higher(by millions)than the number of humans cells,they're indispensable in the food industry,others are fundamental to make medicines and more,what exactly are you asking about ?
What are the natural occurring elements found in organisms on earth?
Otu Reply
some of the naturally occurring elements found in organisms are carbon, nitrogen, hydrogen, oxygen
some of them are carbon, oxygen, nitrogen, hydrogen, phosphorus and sulfur
what is Zika virus?
Somali Reply
Zika virus is a mosquito-borne flavivirus that was first identified in Uganda in 1947 in monkeys. 
intracellular vesicles are found in
Akshay Reply
how do i report widal slide results
Isaac Reply
how food can be used as a substrate for the growth of microorganisms
Ashi Reply
what is the importance of understanding chemistry in the field of microbiology and food technology?
Charmaine Reply
What is different between eukaryotes and prokcaryotic
Abia Reply
main difference is that eukaryotic cells possess membrane bound organelles
prokaryotes are primitive organisms that doesn't contain membrane bound nucleus or any orgenelle while in eukaryotes membrane bound orgenelles and nucleus is present
what s anatomy
jane Reply
its mean body structure, function, and systems
study of internal structure of living things
anatomy is the study structure of the made human
anotomy is the actual study of body internally and externally Which include how itis made. for what and what is need of this
la science anatomie c'est la science qui nous aide à étudier l'homme
Ancestor are they real
Rapheal Reply
yes of course
yes please
50 50, depending on the accuracy of the clan records.
Why protist is not a kingdom of Linnaeus 'S taxonomy?
Neha Reply
I dont know
Linnaeus used Aristotle's criteria for dividing living organisms into kingdoms. Plants are immobile and insensitive, while animals are mobile and sensitive. the creatures he saw through the microscope were mobile, so he attributed them to animals.
name the different types of media use in lab to detect the micro organisms
Shehzadi Reply
The different types of media used in the lab to detect the microorganisms is known as cell culture..
solid media or broth is used to grow and detect microorganisms
what is microbiology
the study of large living organisms
the study of organisms which are micro in range
what is autoclaving?
process for sterilization
is a machine used on the process of sterilisation
what is difference between hot air oven and autoclave as they both are used for sterilization ?
autoclave basically do moist heat sterilization while hot air oven do sterilization by dry heat.....
what is the microbe
which method out of these two is best?
why human have microbe
hi to all
depends on what to sterilize
what is knowledge
well idea
what are the importance of microbiologe to a nurse
Enny Reply
it enable a nurse to know the weight and height of a patient
it helps the nurse in way as to give first aid to patient on which basis doctor will take diagonos
disagree with both of you
Your Own idea
it helps a nurse to be able to counsel a patient /client
cell culture
Types of micro organisms
Jennifer Reply
Who discovered plant and animal cell
Robert Hooke

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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