Cobra Effect · Microbes and the machinery of life
The flasks with necks like swans
How a bent glass neck showed that microbes come from other microbes.
7 cards, read aloud in 2:31, with a test and sources.
For centuries, many people believed that life could arise on its own from non-living matter.
Maggots seemed to appear from rotting meat, and mould from spoiled food. Even after microscopes revealed microbes, some scientists thought these formed freshly in broths. The idea was called spontaneous generation.
In 1859, the naturalist Felix Pouchet reported microbes in boiled hay infusions.
He had boiled them to kill any life, yet microbes still appeared. The French Academy of Sciences offered a prize for experiments that could shed light on the question. Louis Pasteur took up the challenge.
Pasteur boiled broth in flasks whose necks he drew out and bent into an S shape.
Air could still flow in and out of the open neck. But dust and microbes from the air settled in the low bend of the neck. The broth stayed clear, with nothing growing in it.
When he broke off the neck, the broth soon clouded with microbes.
Tipping the flask so the broth touched the dusty bend did the same. So the broth itself could support life. It only needed microbes to arrive. The microbes came from other microbes, carried in the air.
In 1862 he won the prize, and in 1864 he showed his flasks at a public lecture in Paris.
He declared that spontaneous generation would never recover from this simple experiment. A commission of the Academy sided with him. Pouchet’s team withdrew rather than repeat their experiments in front of it.
But Pouchet had not simply been careless.
Hay carries bacterial spores that can survive boiling. That is why his boiled hay infusions really did grow microbes. The spores were only understood in the 1870s, through the work of Ferdinand Cohn and John Tyndall.
So when an experiment seems to settle a question, ask what it actually ruled out.
Pasteur showed that the microbes in his broths came from other microbes. No experiment could prove that life never arises from non-living matter, anywhere, ever. How life first began on Earth is still an open question.
Sources
- Spontaneous Generation, OpenStax Microbiology. A free textbook chapter on the debate, with the flask experiment explained.
- Swan neck flask, Wikipedia. How the flask works, and how it was used.
- A swan necked flask used by Louis Pasteur, Wellcome Collection. A photograph of one of Pasteur’s flasks.
Nearby ideas
- The little animals in a drop of water. How a Dutch cloth merchant with homemade lenses discovered a hidden world of life.
- Germs do not come from nowhere. How a swan-neck flask and a country doctor proved that germs cause disease.
- Lightning in a flask. How sparks in a glass loop made some of the building blocks of life.
- The cells that took in bacteria. How a rejected idea showed that parts of our cells were once free living bacteria.
- The third branch of life. How comparing one molecule revealed a whole domain of life hiding in plain sight.
- The infection with no genes. How a brain disease in sheep led to infectious proteins with no genes at all.