Koch's Postulates - Online Tutor, Practice Problems & Exam Prep

Koch's postulates are a set of four criteria established by Robert Koch to determine if a specific microbe causes a particular disease. The postulates state that the microbe must be present in every case of the disease, isolated and grown in pure culture, cause the same disease when introduced to a susceptible host, and be re-isolated from that host. Limitations include the inability to culture all pathogens and variations in host susceptibility, highlighting the complexities of microbial pathogenesis and the need for ethical considerations in experimentation.

Koch's Postulates

In this video, we're going to begin our lesson on Koch's postulates. Robert Koch was a German physician of the late 1800s who designed a set of postulates, or criteria, that are used to identify if a particular microbe causes a specific disease. Using his postulates, Robert Koch was able to discover that anthrax, a specific disease in cattle and humans, is actually caused by the bacterium Bacillus anthracis. Koch's postulates can be defined as a set of criteria that are used to determine if a particular microbe causes a particular disease.

If we take a look at this image below, you'll notice that we are showing you Robert Koch. Notice Robert Koch is saying, "Hey, my 4 postulates can show us what pathogen is causing your disease." Notice that Robert Koch is saying that to Doctor Klutch Prep. Notice that Doctor Klutch Prep is sweating, has a fever, and is not feeling very well. He is appreciative that Robert Koch can use his postulates to determine if a microbe is causing his disease.

In our next lesson video, we'll be able to talk more about the details of Robert Koch's postulates. So, I'll see you all in that video.

The 4 Koch's Postulates

In this video, we're going to talk more details about Robert Koch's postulates. And so it turns out that there are actually 4 postulates that Robert Koch used to link a specific microbe to a specific disease. Notice down below, we're showing you an image of all 4 of these postulates that are used to determine, once again, if a specific microbe causes a specific disease. The very first postulate states that the microbe must be present in every single case of the disease. It can't be the case that the disease is present in a host, but the microbe is not. If that's the case, then the microbe probably doesn't cause the disease. Notice over here what we are showing you is a mouse that is a diseased mouse, and it's not looking very good over here.

Now the second postulate states that the microbe must be isolated from the diseased host, and it must be grown in a pure culture. Recall that a pure culture is a culture that consists of only one single species, which would be the specific microbe that was isolated from the host. Notice over here we're showing you a syringe going into the diseased host here, and the syringe is extracting the specific microbe. That microbe in the syringe is being grown on a petri dish, which we refer to as a pure culture because it only contains the one specific microbe that was isolated from the diseased host.

Now the third postulate states that the same exact disease must occur when the pure culture that was isolated from the diseased host is introduced into another susceptible host. Notice over here we are taking the Pure Culture and the Pure Culture is being introduced into another mouse, another susceptible host, and so when that happens this mouse must develop the same exact disease.

The fourth postulate states that the microbe must be experimentally isolated, or must be isolated from the experimentally infected host. What that means is that, from this mouse that developed the disease, a syringe must be able to go into that mouse and extract the same exact microbe. These are the 4 postulates that make up Koch's postulates and that must be met in order to conclude that a specific microbe causes a specific disease. If these postulates cannot be met, then the assumption is that the microbe does not cause this specific disease. However, there are some limitations to Koch's postulates that we will talk about more as we move forward in our course. But for now, this here concludes our discussion on Koch's postulates, and we'll get to learn more as we move forward. I'll see you all in our next video.

Chronic wasting disease (CWD) in deer, elk, and moose is caused by a prion. Which of Koch's postulates would not work in regards to identifying the cause of CWD?

• A. Postulate 1.
• B. Postulate 2.
• C. Postulate 3.
• D. Postulate 4.

Koch stressed the importance of working with ______ to formulate his postulates of disease.

• A. Anaerobic culture.
• B. Enriched culture.
• C. Pure culture.
• D. Mixed culture.
• E. Abstract culture.

Koch's postulates established:

• A. Evidence against the idea of spontaneous generation of life.
• B. That bacterial cells are the fundamental units of all life.
• C. That specific diseases are caused by specific pathogens.
• D. That virulence factors are the characteristics of pathogens which cause disease.

Which of the following is not one of Koch's postulates?

• A. The same pathogen must be present in every single case of the disease.
• B. The pathogen from pure culture must cause the disease when inoculated into a healthy, susceptible test subject.
• C. The pathogen must be isolated and grown in pure culture from diseased test subject.
• D. The disease must be transmitted from a diseased test subject to a susceptible test subject via direct contact.

Limitations to Koch's Postulates

So while Koch's postulates serve an important role in determining the cause of disease, there are limitations to Koch's postulates. In this video, we're going to be focusing on some of the limitations to Koch's postulates. If you take a look at our image down below, notice that towards the top of our image what we have is Koch's postulates as we discussed them in our last lesson video. And then notice down below we have some of the limitations to Koch's postulates. Recall that the second of Koch's postulates states that a microbe must be isolated from the diseased host and grown in pure culture. A limitation for this particular postulate is that unfortunately not all pathogenic microbes can actually grow on laboratory media, and so we are not able to grow all pathogenic microbes in pure culture. That's a huge limitation because, for example, obligate intracellular pathogens, they cannot be grown in pure culture because they can only grow in the presence of a host cell. And there are many other organisms as well that again we do not know exactly how to grow them in a lab because we don't know exactly what conditions they need in order to grow them in the lab. This is a limitation to Koch's postulates. Not all of the pathogenic microbes can be grown in pure culture. Now recall that the third of Koch's postulate states that the same exact disease must occur when the pure culture is introduced into a susceptible host. There are some limitations to this one as well because some pathogenic microbes actually do not cause disease in all infected individuals, and that's because not all test subjects are going to be susceptible to the disease. What you'll get is some individuals will have stronger immune systems, others will have weaker immune systems and that can lead to asymptomatic individuals. Like for example, individuals that may have COVID-19 may be asymptomatic and therefore may not display any signs or symptoms of the disease. Now, the fourth of Koch's postulates states that the microbe must be isolated from the experimentally infected host and there are limitations for this as well because there are plenty of ethical and safety concerns when it comes to human and animal testing and this can make the results unreliable especially for human diseases. This here concludes our brief discussion about the limitations to Koch's postulates and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.

Which of the following would Koch's postulates not be able to explain?

• A. Diseases caused by viruses.
• B. Diseases with asymptomatic carriers.
• C. Diseases that have long latency (dormant) periods after infection.
• D. All of the above.

In two of Koch's postulates (#2 and #3), a pure culture of the pathogen is required. Which of the following would not be a possible consequence of using a contaminated culture?

• A. The disease cannot be attributed to a single microbe since the culture is contaminated with other microbes.
• B. The contaminating microbe may also cause disease in the test subject in addition to the microbe of interest.
• C. The contaminating microbe might outcompete the microbe of interest for nutrients, killing to microbe of interest.
• D. Even with a contaminant present, if the microbe of interest is present, it should infect the test animal.

Why would Koch's postulates not be able to identify that the SARS-CoV-2 virus causes the Covid-19 disease?

• A. Some individuals infected with SARS-CoV-2 do not show symptoms.
• B. The SARS-CoV-2 virus is not present in all cases of Covid-19.
• C. Viruses cannot be cultured on laboratory media.
• D. Some individuals have weaker immune systems and are more susceptible to the SARS-CoV-2 virus.
• E. A and B.
• F. A, C, and D.
• G. C and D.

Here’s what students ask on this topic:

What are Koch's postulates and why are they important?

Koch's postulates are a set of four criteria established by Robert Koch to determine if a specific microbe causes a particular disease. The postulates are: 1) The microbe must be present in every case of the disease. 2) The microbe must be isolated from the diseased host and grown in pure culture. 3) The same disease must occur when the pure culture is introduced into a susceptible host. 4) The microbe must be re-isolated from the experimentally infected host. These postulates are important because they provide a systematic method to identify the causative agents of infectious diseases, which is crucial for diagnosis, treatment, and prevention.

What are the limitations of Koch's postulates?

While Koch's postulates are foundational in microbiology, they have limitations. Not all pathogenic microbes can be grown in pure culture, such as obligate intracellular pathogens that require a host cell to grow. Additionally, some microbes do not cause disease in all infected individuals due to variations in immune response, leading to asymptomatic cases. Ethical and safety concerns also limit the ability to experimentally infect humans or animals, making it difficult to fulfill all postulates. These limitations highlight the complexities of microbial pathogenesis and the need for alternative methods in some cases.

How did Robert Koch use his postulates to identify the cause of anthrax?

Robert Koch used his postulates to identify Bacillus anthracis as the causative agent of anthrax. He observed that the bacterium was present in all cases of the disease. He then isolated B. anthracis from diseased animals and grew it in pure culture. When he introduced the pure culture into healthy, susceptible animals, they developed anthrax. Finally, he re-isolated the same bacterium from these experimentally infected animals, fulfilling all four of his postulates and conclusively linking B. anthracis to anthrax.

Can Koch's postulates be applied to all diseases?

No, Koch's postulates cannot be applied to all diseases. Some pathogens cannot be grown in pure culture, such as viruses and certain bacteria that require specific host conditions. Additionally, some diseases are caused by multiple pathogens or involve complex interactions between the host and microbe, making it difficult to apply the postulates. Ethical concerns also prevent the experimental infection of humans in many cases. Therefore, while Koch's postulates are a valuable tool, they are not universally applicable, and alternative methods may be needed to identify the causes of some diseases.

What is the significance of pure culture in Koch's postulates?

The concept of pure culture is crucial in Koch's postulates because it ensures that the microbe being studied is the only organism present, eliminating the possibility of contamination or interference from other microbes. By isolating and growing the microbe in pure culture, researchers can accurately link it to the disease in question. This step is essential for demonstrating that the microbe alone is responsible for causing the disease, thereby fulfilling the second postulate and providing a clear, uncontaminated sample for further testing in the subsequent postulates.