Pasteur and Koch: Investigating the Causes of Infectious Disease

Louis Pasteur

The work of both Louis Pasteur and Robert Koch established conclusively that microorganisms are the cause of infectious diseases.

Prior to the work of Louis Pasteur (1822-1895) and Robert Koch (1843-1910), many scientists promoted the ‘spontaneous generation’ theory as a way of explaining the appearance of microbes in spoiling food and rotting organic matter. This theory, championed by naturalists such as John Turberville Needham and Georges- Louis Leclerc, suggested that microbes and other life forms appeared from nowhere and proceeded to multiply on any available food source.

Pasteur’s studies, however, inferred that microbes such as bacteria, fungi,viruses and protozoans were already present in organic material and multiplied when conditions became favourable to them. This ‘germ theory’ explanation for food spoiling, fermentation and disease was reinforced when Robert Koch’s famous postulates helped to prove how a specific microorganism could cause a particular disease.

Pasteur and Fermentation

Despite his initial work in the field of stereochemistry, Louis Pasteur is more commonly recognised for his contributions to microbiology. As Professor of Chemistry at the University of Lille, Pasteur was asked to investigate the production problems of some of the local beer and wine industries. After careful microscopic investigations, he concluded that a specific microorganism was responsible for different types of fermentation.

Wine and beer, for instance, were the fermentation products of members of the yeast genus Saccharomyces, whereas their souring was due to the presence of bacteria such as Lactobacillus and Acetobacter. Pasteur’s subsequent realization that heating the wine or beer to around 50- 60 degrees Celsius could destroy these microbes helped to solve many of the spoilage problems in these industries.

The Link Between Microbes, Food Spoiling and Disease

This process, which became known as ‘pasteurisation’, is still used today in the food and milk industries. Although Pasteur’s initial intention was to prevent spoilage in wine and beer, pasteurization can also help to remove disease-causing organisms that may be present in food. Pasteurising milk, for instance, can destroy microbes that cause tuberculosis, brucellosis, diphtheria, scarlet fever and salmonella poisoning.

Pasteur’s famous ‘meat broth’ experiment in 1862 further strengthened the idea that food spoiling is a result, rather than a side effect, of microbial contamination. In this experiment he used two flasks containing equal amounts of meat broth. One flask had an open neck while the other had a curved, ‘swan-like’ neck that could trap any particles in the air before they reached the broth.

After boiling the broth in both flasks and leaving them for a period of time, he found that only the broth in the open-necked flask had become spoiled. This suggested that something from the air had caused the contamination, thus disproving the theory of spontaneous generation.

Pasteur first demonstrated the link between microbes and disease in 1865 when he isolated the microscopic parasite (Nosema bomycis) that was killing silkworms throughout France. By recommending that only uninfected eggs be selected for farming, he helped to save the industry in France and other parts of Europe.

Further Diseases and the Germ Theory

Pasteur later applied his ‘germ theory’ to other diseases, identifying the microbes responsible for anthrax, cholera, tuberculosis and rabies. Moreover, in 1879 he discovered that chickens injected with an attenuated (weakened) form of the chicken cholera bacterium were protected against contracting the more virulent form of the disease.

This led to Pasteur’s development of a similar vaccine for sheep anthrax in 1881. A public trial of this vaccine met with resounding success and helped to resolve the anthrax epidemic occurring in France at the time. In 1879 Pasteur also successfully vaccinated a nine- year- old boy with an attenuated form of the rabies virus, heralding the first inroads into preventive medicine.

Robert Koch’s Postulates

Pasteur had shown that microorganisms were present whenever food spoiled or an organism was diagnosed with an infectious disease. He also demonstrated that these microbes did not ‘spontaneously generate’, but were instead ubiquitous in the air and living things, simply waiting for the ideal conditions to reproduce themselves. Pasteur’s insistence on hygienic conditions in the workplace consequently reduced contamination and spoiling in wine and beer and paved the way for the use of aseptic techniques in medical procedures.

However, Pasteur had never decisively linked a microorganism to a particular disease. Koch, a medical doctor, had the necessary background to develop a series of steps that could, indeed, prove this. Initially working with anthrax in sheep, Koch further developed the findings of Casimir Davaine, who had shown that healthy sheep could contract anthrax if injected with the blood of infected animals.

Koch extended this finding by successfully isolating and culturing the anthrax bacillus (Bacillus anthracis) and reintroducing it into healthy sheep and mice. As he predicted, these animals later developed the symptoms of anthrax. Koch achieved similar results after conducting tests with the bacteria that cause tuberculosis and cholera. In 1890 this work led him to present four basic steps required to determine that a particular microbe caused a given disease. Later known as ‘Koch’s Postulates’, they consist of the following rules:

•                The microorganism must be present in all diseased organisms.

•                The microorganism must be capable of being isolated and identified.

•                When healthy animals are inoculated with the microorganism they must develop symptoms of the original disease.

•                The microorganism must be recoverable from these newly diseased animals. It must then be isolated and identified as being the same as the microbe in the original infected animals.

The Legacy of Pasteur and Koch

By 1900, Koch had identified the germs that caused twenty -one diseases, and had developed an improved method of staining cells for use in microscopic studies. He also demonstrated that microbes caused wounds to become septic, making the link, once again, between microbes and infectious diseases.

Both Koch’s efforts, and the work of Pasteur, contributed to the preventive medicine, sanitation and hygiene practices we take for granted today. The spontaneous generation myth had been dispelled conclusively.

References

•                BBC, 2012, ‘Louis Pasteur

•                Encyclopædia Britannica Inc., 2012.'Louis Pasteur.' Encyclopædia Britannica Online.

•                Hani, 2010, ‘Discovery of Pasteurisation’, experiment-resources.com

•                History Learning Site, 2012, ‘Robert Koch’, historylearningsite.uk

•                Shurtleff , W. and Aoyagi , A., 2007, ‘A brief History of Fermentation, East and West’, Soyinfo Centre, California