The Germ Theory of Disease: Experiments, and Applications

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.

Louis Pasteur first introduced the germ theory of disease in the 19th century. It was further explained by other scientists such as Robert Koch and Joseph Lister. Until the 19th century, miasma theory was the predominant theory.

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.
• Anton Van Leeuwenhoek, the first microbiologist to observe microorganisms under a microscope, also supported the germ theory of disease. Furthermore, in the 1700s, Richard Bradley postulated that diseases were caused by microorganisms only visible 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.

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

In 1856, while studying the causes of wine and beer spoilage, Pasteur discovered the properties of fermentation by microorganisms. He demonstrated with a swan neck experiment that airborne microbes, not spontaneous generation, were the cause of food spoilage. Therefore, he suggested that if microbes are responsible for food spoilage and fermentation, they could be responsible for causing infections. This speculation was the foundation of the germ theory of disease. In the experiment conducted by Pasteur, freshly prepared boiled broth was exposed to the air under the following conditions;

• The broth was kept in a vessel containing a filter to restrict the entry of air particles.
• The broth was kept in a vessel without a filter and was disclosed to the room air.
• Lastly, the broth was kept in a vessel exposed to room air with the help of a long tube. This was to prevent the access of dust particles.

As a result, Pasteur detected that the growth of microorganisms only occurred in a broth disclosed to the room air without a filter.

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.