“Microorganisms too small to be seen or felt exist everywhere in the air and cause disease and food spoilage.”

  • Who Discovered it?: Louis Pasteur
  • Year of Discovery: 1856

How was it Discovered?


In the fall of 1856, 38-year-old Louis Pasteur was in his fourth year as Director of Scientific Affairs at the famed Ecole Normale in Paris. It was an honored administrative position. But Pasteur’s heart was in pure research chemistry and he was angry.

Many scientists believed that microorganisms had no parent organism. Instead, they spontaneously generated from the decaying molecules of organic matter to spoil milk and rot meat. Felix Pouchet, the leading spokesman for this group, and had just published a paper claiming to prove this thesis.

Pasteur thought Pouchet’s theory was rubbish. Pasteur’s earlier discovery that microscopic live organisms (bacteria called yeasts) were always present during, and seemed to cause, the fermentation of beer and wine, made Pasteur suspect that microorganisms lived in the air and simply fell by chance onto food and all living matter, rapidly multiplying only when they found a decaying substance to use as nutrient.

Two questions were at the center of the argument. First, did living microbes really float in the air? Second, was it possible for microbes to grow spontaneously (in a sterile environment where no microbes already existed)?

Pasteur heated a glass tube to sterilize both the tube and the air inside. He plugged the open end with guncotton and used a vacuum pump to draw air through the cotton filter and into this sterile glass tube.

Pasteur reasoned that any microbes floating in the air should be concentrated on the outside of the cotton filter as the air was sucked through it. Bacterial growth on the filter indicated microbes floating freely in the air. Bacterial growth in the sterile interior of the tube meant spontaneous generation.

After 24 hours the outside of his cotton wad turned dingy gray with bacterial growth while the inside of the tube remained clear. Question number 1 was answered. Yes, microscopic organisms did exist, floating, in the air. Any time they concentrated (as on a cotton wad) they began to multiply.

Now for question number 2. Pasteur had to prove that microscopic bacteria could not spontaneously generate.

Pasteur mixed a nutrient-rich bullion (a favorite food of hungry bacteria) in a large beaker with a long, curving glass neck. He heated the beaker so that the bullion boiled and the glass glowed. This killed any bacteria already in the bullion or in the air inside the beaker. Then he quickly stoppered this sterile beaker. Any growth in the beaker now had to come from spontaneous generation.

He slid the beaker into a small warming oven, used to speed the growth of bacterial cultures.

Twenty-four hours later, Pasture checked the beaker. All was crystal clear. He checked every day for eight weeks. Nothing grew at all in the beaker. Bacteria did not spontaneously generate. Pasteur broke the beaker’s neck and let normal, unsterilized air flow into the beaker. Seven hours later he saw the first faint tufts of bacterial growth. Within 24 hours, the surface of the bullion was covered.

Pouchet was wrong. Without the original airborne microbes floating into contact with a nutrient, there was no bacterial growth. They did not spontaneously generate.

Pasteur triumphantly published his discoveries. More important, his discovery gave birth to a brand new field of study, microbiology.


Fun Facts: The typical household sponge holds as many as 320 million disease-causing germs.


Microscopic organisms exist that can not be seen by the human eye.

  • Who discovered it: Anton van Leeuwenhoek
  • Year of Discovery: 1680

How was it Discovered?

Anton van Leeuwenhoek was born in 1632 in Delft, Holland. With no advanced schooling, he was apprenticed as a cloth merchant and assumed that buying and selling cloth would be his career. But van Leeuwenhoek was curious about the world and interested in mathematics.

Completely self-taught, he learned enough math to moonlight as a surveyor and read what he could about the natural world around him. He never learned any language other than Dutch, so he was never able to read any of the scientific papers and research (all written in Latin or French).

Microscopes existed in Holland by 1620. Christian Huygens and Robert Hooke were the first two scientists to make scientific use of microscopes. Both designed and built two-lens microscopes (two ground glass lenses inside a thin metal barrel).

In 1657 van Leeuwenhoek looked through his first microscope and was fascinated. He tried a two-lens microscope but was disappointed by its distortion and low resolution. When he built his first microscope, he used a highly curved single lens to gain greater magnification.

By 1673 van Leeuwenhoek had built a 270-power microscope that was able to see objects only one-one-millionth of a meter in length. Van Leeuwenhoek remained very secretive about his work and never allowed others to see his microscopes or setup.

Van Leeuwenhoek started his microscopic studies with objects he could mount on the point of a pin, a bee’s mouthparts, fleas, human hairs, etc. He described and drew what he saw in precise detail. By 1674 he had developed the ability to focus on a flat dish and turned his attention to liquids—water drops, blood cells, etc.

Those 1674 studies were where he made his great discovery. He discovered a host of microscopic protozoa (bacteria) in every water drop. He had discovered microscopic life, invisible to the human eye.

Van Leeuwenhoek expanded his search for these unseeably small creatures and found them everywhere: on human eyelashes, on fleas, in dust, and on skin. He drew and described these tiny creatures with excellent, precise drawings.

Each drawing often took days to complete. As an amateur, Van Leeuwenhoek had to work at his science in the evenings and early morning hours when not at work. Embarrassed by his lack of language skills and by his poor spelling (even in Dutch), van Leeuwenhoek felt hesitant to publish any articles about his wondrous findings.

Beginning in 1676, he agreed to send letters and drawings to the Royal Society of London. They had them translated into English. That extensive collection of letters (written and collected over many decades) formed the first and best map of the microscopic world.

What van Leeuwenhoek observed shattered many scientific beliefs of the day and put him decades—if not centuries—ahead of other researchers.

He was the first to claim that bacteria cause infection and disease. (No one else believed it until Pasteur proved it in 1856.) Van Leeuwenhoek saw that vinegar killed bacteria and said that it would clean wounds. Again, it was two centuries before his belief be came standard medical practice.

It was also 200 years before anyone built a better microscope. But with his marvellous microscope, van Leeuwenhoek discovered the critically important microscopic world.