Germ theory in 60 seconds: ideas that changed the world

A new understanding of bacteria revolutionised medicine in the 19th century

Illustration of Louis Pasteur, Joseph Lister and Robert Koch
(Image credit: Illustrated / Getty Images)

In this series, The Week looks at the ideas and innovations that permanently changed the way we see the world. 

Germ theory in 60 seconds

Germ theory refers to the principle that "certain diseases are caused by the invasion of the body by microorganisms" such as bacteria, parasites, or viruses, according to the Encyclopedia Britannica. The physical existence of germs was observed some two hundred years before the development of germ theory, but their origin and effect remained largely unknown until the latter half of the 19th century. 

Although earlier scientists had expressed similar ideas, Louis Pasteur was the first to provide scientific proof for the principles of germ theory, initially with relation to how microbes affect the fermentation of alcohol.  

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In the 1870s, he applied this knowledge to the field of medicine. After demonstrating that the decay of meat was caused by microbes, Pasteur "argued that this could explain disease as well as decay, claiming that disease was caused by the multiplication of germs in the body", said the Science Museum 

His work launched the field of bacteriology, revolutionised our understanding of how diseases are transmitted and – crucially – demonstrated the importance of a sterile environment for treating the sick and carrying out surgery.

How did it develop?

 In the 1860s, Pasteur - already a celebrated scientist in his native France - was enlisted to help the country’s winemakers better understand the process of fermentation. His research in this field uncovered new findings about microbes, not least the effect of oxygen on bacteria.

Winemakers were quick to adopt Pasteur’s system of applying just enough heat to kill off harmful bacteria in food and drink without affecting the taste - a process he called pasteurisation.

Nevertheless, for several years "there remained significant doubt about the causative role of bacteria in disease" within the medical establishment, according to John Booss and Alex C. Tselis in their "Handbook of Clinical Neurology".

Until the middle of the 19th century, these microorganisms were generally assumed to be a by-product of disease rather than its cause. The prevalent theory of "spontaneous generation" held that organisms could simply appear - for instance, that maggots could arise from dead flesh.

In a famous experiment, Pasteur disproved this theory by showing that while boiled beef broth became cloudy when exposed to air - indicating contamination - beef broth boiled in a flask with a long, curved neck to prevent any external particles from coming into contact with the liquid remained clear. "These experiments proved that there was no spontaneous generation, since the boiled broth, if never reexposed to air, remained sterile," according to the Encyclopedia Britannica.

German physician Robert Koch laid to rest any lingering doubts about germ theory. In the 1870s, he developed new laboratory techniques that enabled scientists to isolate and cultivate specific bacteria.

Thanks to Koch’s advances, "causes of significant bacterial diseases" such as tuberculosis and cholera could be identified and studied for the first time, wrote Booss and Tselis.

English physician Joseph Lister had noted back in the 1850s that cleaning wounds with a caustic substance appeared to reduce the risk of infection, but his claims went largely ignored until the work of Pasteur and Koch gained widespread acceptance.

With the principle of germ theory now recognised, Lister’s theories about the use of antiseptic to kill bacteria also gained acceptance. In 1871, he was even asked to treat Queen Victoria, who was suffering from an abscess in her arm.

"Lister’s methods transformed surgery from a butchering art to a modern science," wrote Lindsey Fitzharris in "The Butchering Art: Joseph Lister’s Quest to Transform the Grisly World of Victorian Medicine". His methods have since saved countless lives.

How did it change the world?

"The conclusive demonstration that certain diseases, as well as the infection of surgical wounds, were directly caused by minute living organisms… effected a complete revolution in the practice of surgery," says the Encyclopedia Britannica.

"Without antiseptic and aseptic environments and procedures, along with antibiotics and antiviral and antifungal agents, modern medicine would be nearly impossible," said Joel and Salomao Faintuch in "Microbiome and Metabolome in Diagnosis, Therapy, and other Strategic Applications".

Furthermore, without the knowledge of germ theory, hospitals would be little better than medieval almshouses, "and the healthcare profession would not perform much better than at the time of alchemists, herbalists, and barber surgeons", they added.

Germ theory also provided the launch pad for the field of medical microbiology. Koch’s advances in laboratory techniques enabled scientists to identify the pathogens responsible for deadly infectious diseases – the vital first step towards developing the vaccines and medicines that have eradicated many of these diseases from countries across the globe.

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