Germ Theory of Disease: Experiments and Key Figures

For most of human history, no one knew what caused disease. Bad air, imbalanced humors, punishment, or spontaneous generation from rotting matter were the accepted explanations. Then, in a few remarkable decades of the nineteenth century, a handful of scientists proved that many diseases are caused by living microorganisms too small to see, and that single idea reshaped medicine, surgery, and public health more than almost any other. This article is about how they proved it: the experiments, the logic, and the people behind the germ theory of disease.

The germ theory of disease state that ‘the invasion of microorganisms such as bacteria, viruses, fungi, or protists causes infectious diseases in human beings’. These microorganisms are tiny to be seen by our naked eyes and require a microscope for visualization. These microorganisms can enter the host body through various routes, like the mouth, nose, and skin. Once inside the body, they can multiply and cause disease by damaging cells, producing toxins, or triggering an immune response.

The germ theory of disease is most often credited to Louis Pasteur, who advanced and popularized it in the 19th century, and to Robert Koch, who provided the decisive experimental proof that specific microbes cause specific diseases. Joseph Lister then applied it to surgery. The idea had older roots (see "Who proposed the germ theory" below), but these three figures established it as science. Before them, the miasma theory, which blamed disease on "bad air," was the dominant explanation.

History of Germ Theory of Disease

Until the 19th century may scientists made assumptions about the causes of diseases that are as follows;

• In ancient Greece, the belief was disease spread by contagious ‘seeds’ or food products instead of contact with an infected individual. Further, such seeds might reside in the host body, causing a successive disease relapse after a specific duration. Diseases like the Black Death that occurred in the European population during the Middle age were believed to be originated like this.

• This concept was later revisited by the scientists of middle age (e.g., Girolamo Fracastoro). Where they added that seed-like spores might be transferred between the individual through direct contact. Also the diseases were transferred by exposure to contaminated clothing, or through the air.

• It was believed that living organisms could emerge from non-living matter, known as the theory of spontaneous generation. In the 1600s, the concept of disease caused by spontaneous generation was invalidated by the work of Francesco Redi. Redi’s experimental findings are as follows;

• Jar 1: Meatloaf and eggs exposed to the air without lid.

  • Result: Formation of maggots covering the egg and meatloaf was found.

• Jar 2: Meatloaf and eggs tightly sealed with a lid.

  • Result: No formation of maggots.

• Jar 3: Meatloaf and eggs without lids but covered with gauze.

  • Result: Maggots on the top of the gauze.

• Based on the above findings, Redi disproves the spontaneous generation theory.

• Antonie van Leeuwenhoek, the first person to observe microorganisms under a microscope in the 1670s, did not himself connect microbes to disease (germ theory did not yet exist), but his discovery that a world of tiny living organisms exists was an essential foundation for it. Later, in the 1700s, Richard Bradley proposed that diseases were caused by microorganisms visible only through a microscope. Marcus Antonius Von Plenciz later supported this concept. Further, Von Plenciz also described the ubiquitous presence of the microscopic organism.

• Although support for the germ theory of disease began earlier, it was only proved by Louis Pasteur and Robert Koch through laboratory research.

Who proposed the germ theory of disease?

This is one of the most searched questions about germ theory, and it does not have a single tidy answer, because the theory was built over centuries by different people doing different things.

• Proposed the idea (contagion): the notion that tiny transmissible agents cause disease is old. In 1546, the Italian scholar Girolamo Fracastoro proposed that disease spreads by tiny "seeds" (seminaria) passed between people, by contact, or through the air. This is often cited as the first clear statement of a contagion idea, long before microbes could be seen.
• Advanced and popularized the theory: Louis Pasteur, in the 1860s, provided the experiments (disproving spontaneous generation) and the argument that made germ theory scientifically credible. He is the name most people associate with germ theory.
• Proved it for specific diseases: Robert Koch, from 1876, gave the first rigorous experimental proof that a specific microbe causes a specific disease (anthrax), and set out the logical criteria (Koch's postulates) for proving causation.

So the honest answer depends on the exact question: Fracastoro proposed the idea, Pasteur advanced and popularized it, and Koch proved it. In exams, if asked simply "who proposed the germ theory of disease," the expected answer is usually Louis Pasteur (with Robert Koch credited for proving it).

Experiments in Support of the Germ Theory of Disease

The actions and observations of Ignaz Semmelweis, Joseph Lister, and John Snow would retrospectively be acknowledged as contributing to the acceptance of the germ theory of disease. However, the laboratory research of Louis Pasteur in the 1860s and Robert Koch in the following decades provided scientific proof supporting the germ theory of disease.

Contributions by Louis Pasteur to the Germ Theory of Disease

Contributions of Louis Pasteur: the swan-neck flask experiment

In the 1850s, while investigating why wine and beer spoiled, Pasteur showed that microorganisms cause fermentation. This led him to a bigger idea: if microbes spoil food, microbes might also cause disease. But to make that case, he first had to demolish a rival explanation, spontaneous generation, the belief that life (including microbes) arises on its own from non-living matter.

He did this with the famous swan-neck flask experiment (1859–1862):

1. He boiled nutrient broth in a flask to sterilize it, killing any microbes present.
2. The flask had a long neck bent into an S-shape (swan neck). This let air flow in freely, but dust and airborne microbes were trapped in the bend of the neck and could not reach the broth.
3. The broth stayed clear and sterile for a long time, even though it was exposed to air. This showed that air alone does not generate life.
4. The decisive step: when Pasteur tilted the flask so the broth ran up into the bend and touched the trapped dust, the broth quickly became cloudy with microbial growth.

The tilt was the proof. If spontaneous generation were true, the broth would have spoiled as soon as it met air, regardless of the dust. Instead, it spoiled only when it touched the dust carrying microbes. Microbes came from the environment; they did not arise on their own. Pasteur summed it up as "Omne vivum ex vivo", life comes only from life.

By disproving spontaneous generation, Pasteur cleared the way for germ theory: if microbes must come from other microbes, then the microbes found in disease must have come from outside, not spontaneously appeared, and could be the cause of that disease.

Figure: Experiment done by Louise Pasture that supports germ theory of disease

Contributions of Robert Koch: proving a specific microbe causes a specific disease

Pasteur made germ theory credible; Robert Koch made it certain for individual diseases. In 1876, Koch provided the first rigorous proof that one specific microbe causes one specific disease, using anthrax:

1. He examined the blood of cattle and sheep that had died of anthrax and found the same rod-shaped bacteria (Bacillus anthracis) in every case.
2. He grew these bacteria in pure culture, away from the animal.
3. He injected the cultured bacteria into healthy animals, and they developed anthrax and died.
4. He re-isolated the same bacteria from these newly infected animals and confirmed they were identical to the originals.

This chain, from a diseased animal, to a pure culture, to a new infection, and back to the same microbe, was the first airtight demonstration that a particular bacterium causes a particular disease. From this logic, Koch formulated his famous criteria.

Koch's postulates

From his anthrax work, Koch formulated four criteria for proving that a specific microbe causes a specific disease: the microbe must be present in every case, isolated and grown in pure culture, able to reproduce the disease in a healthy host, and re-isolated from that host unchanged. These criteria gave germ theory its experimental backbone.

For the four postulates in full, their limitations, and the modern molecular version, see our dedicated article on Robert Koch and Koch's postulates.

Contribution by Robert Koch to Germ Theory of Disease

Robert Koch stated that a particular organism causes each specific disease. In his experiment, he isolated anthrax from a diseased host and experimented on it. He then formulated some postulates known as ‘Koch postulates’ based on the following four rules;

1. The microorganism must exist in every case of the disease but absent from the healthy host.
2. The suspected microorganism is isolated and grown in pure culture from diseased individuals.
3. The isolated organism, in pure culture, when inoculated in suitable laboratory animals, should produce a similar disease.
4. The same microorganisms must be isolated again in pure culture from the lesions produced in experimental animals.

Applications of Germ Theory of Disease

The germ theory of disease has a dominant influence on medical and public health, leading to the development of several effective treatments for preventing and treating infectious diseases. Therefore, some of the applications of it are as follows;

1. Discovery of antibiotics: The germ theory of disease has led to the finding and development of antibiotics that help to kill or inhibit the bacteria causing infectious diseases in humans.
2. Discovery of vaccine: The germ theory of disease has led to the discovery of a vaccine that is prepared by using dead, live, or inactive microorganisms to treat infectious diseases in a host.
3. Sterilization: The germ theory of disease has led to the development of techniques to sterilize medical equipment in order to prevent hospital-acquired infections in patients.
4. Personal hygiene: The germ theory of disease has also helped to increase awareness regarding personal hygiene, like hand washing, covering the mouth while sneezing, etc., to prevent the spread of infectious diseases.
5. In Epidemiology: The main content of epidemiology is rooted in the germ theory of disease that helps to trace the source or origin of infectious diseases.

Limitations of Koch Postulates

Although Koch’s postulates were developed as a general guideline for the identification of infectious causes of disease, there are some of the limitations of it that could not be resolved at a time, i.e.;

• Some pathogens cannot be cultured: The first postulate requires the pathogen be present in all cases of disease and absent from healthy individuals, but certain bacteria like Treponema pallidum and Mycobacterium tuberculosis, causative agent of leprosy and tuberculosis, respectively, and viruses cannot be cultured in a laboratory that makes difficult to identify as the causative agent of disease.
• Some diseases have various causes: According to the second postulate, the pathogen should be isolated from the host and grown in pure culture, but some disorders might have multiple reasons. Therefore, isolating a single pathogen might not be sufficient to determine the cause of the disease.
• Ethical concern: The third number of postulates requires that isolated pathogens must cause disease when introduced into a healthy individual. Infecting healthy animals is against the moral right.
• Chronic diseases: The fourth postulate requires the pathogen to be re-isolated and grown in pure culture. However, some pathogens like Mycobacterium tuberculosis can persist for an extended time, making it difficult to isolate the pathogen and establish its role in causing disease.
• Postulates are not universally applicable: Koch’s postulates were developed for bacterial pathogens and might not be relevant to viruses or prions.

References

1. Parija S.C. (2012). Textbook of Microbiology & Immunology.(2 ed.). India: Elsevier India.
2. Sastry A.S. & Bhat S.K. (2016). Essentials of Medical Microbiology. New Delhi : Jaypee Brothers Medical Publishers.