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The Germ Theory of Disease: Definition & Louis Pasteur

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  • 0:03 Bacterial Colonization…
  • 1:20 Spontaneous Generation
  • 2:03 Louis Pasteur
  • 3:41 Germ Theory of Disease
  • 5:30 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

In this lesson, we will look at one of the most influential experiments in the field of microbiology. We will learn how Louis Pasteur disproved the theory of spontaneous generation which helped lead to the development of the germ theory of disease.

Bacterial Colonization of Surfaces

I want you to try an experiment. This might take a while for results to develop, so be prepared to wait. I want you to buy a nice steak, open it, place it on your kitchen counter, and leave it alone for a week. Don't cover it, cook it, toss it out, or disturb it in any way. Come back in a week so we can discuss the results.

Unless you live alone, in a house you own, way out in the country, I wouldn't recommend actually doing this experiment. But, let's pretend we did. What kind of observations do you think we could make? First off, the steak probably smells really bad. There are probably flies and maggots crawling all over the surface. There may be rodent droppings and gnaw marks. If you zoomed in on the surface of the meat and examined it under a microscope, you would no doubt see millions of bacterial cells. But where did this massive population of microorganisms come from?

For hundreds of years, this question was easy to answer. The organisms formed from the dead meat itself. As in, actually arose from the matter in the meat. This sounds ridiculous now, but this is what people believed for hundreds of years - until Louis Pasteur came along.

Spontaneous Generation

In the 1700s and 1800s, variations of this steak experiment were being conducted to prove, or disprove, the theory of spontaneous generation. Spontaneous generation states that living organisms can arise from inanimate, nonliving matter. This was the theory used to explain why food that was left out would eventually be covered in microbes, maggots, flies, and rodents. These organisms were created by, then arose from, the food itself.

In France in 1822, a future scientist was born who would refute the spontaneous generation hypothesis and lay the groundwork for how people would approach medicine and microbiology for generations.

Louis Pasteur

Louis Pasteur was a firm believer in biogenesis - the idea that living organisms could only come from other living organisms. Pasteur observed that there were tiny organisms present in the air around us that resembled the tiny organisms present on the putrefying meat. He concluded that the organisms settled out of the air and infected the meat, not that the meat spawned them. Surrounded by doubters, he decided to prove his theory using meat broth.

It had already been established that boiling broths will prevent them from spoiling, as long as they are protected from exposure to the air afterwards. Spontaneous generation supporters claimed that boiling broth in a sealed flask only proved that fresh air was required. To put this objection to the test, Pasteur designed his own flask with a long, curved neck. This swan-necked flask was open to the air, but had a low curved dip in the neck. This dip caught any microbes that settled into the open end of the flask before they could reach the broth.

When Pasteur boiled broth in his flask, it remained exposed to fresh air but did not develop microbes afterwards when left sitting for days to weeks and even longer. To test his theory further, Pasteur tipped the flask until the broth ran into the curved neck, contaminating it with the organisms that had settled into the dip. In a very short time, the broth turned cloudy due to the growth of the airborne bacteria. This simple, yet brilliant experiment helped shift the way people thought about disease.

The Germ Theory of Disease

Before Pasteur disproved spontaneous generation, he decided to determine why some bottles of wine soured over time. He observed wine that had soured and compared it to wine that had not. He determined that all soured wines contained a large number of cells. Yeast cells are required for wine fermentation, so even good wine would have yeast cells. But, sour wine was full of many smaller cells that were not yeast. Pasteur reasoned that sour wines had been contaminated with microbes, and these contaminating microbes were causing the poor quality.

In 1857, Pasteur published his idea that tiny microorganisms, called germs, were responsible for the sour wine. He also implied that these germs could play a role in human illnesses. Five years later, he disproved spontaneous generation. Armed with the results of these two experiments, as well as information from the work of many other scientists, Pasteur helped contribute to the germ theory of disease. This theory states that some infectious diseases are caused by tiny microorganisms invading a host organism.

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