“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.