Pasteur's refutation of spontaneous generation

By the mid-nineteenth century, naturalists had argued for centuries over spontaneous generation, an idea that went back to Aristotle. The question mattered practically as well as philosophically: if microorganisms could arise from non-living matter, then sterilized food or surgical wounds might become contaminated regardless of precautions. Felix Pouchet, a French naturalist, had published new experiments in 1859 claiming to demonstrate spontaneous generation in carefully prepared broths, and in 1860 the Academie des Sciences offered its Alhumbert prize for experiments that could settle the matter. Louis Pasteur, then studying fermentation at the Ecole Normale Superieure in Paris, entered the contest.

The flask design came from the chemist Antoine-Jerome Balard, who had brought Pasteur into his laboratory at the Ecole Normale. Pasteur boiled broth in glass flasks fitted with long necks that he had drawn out and bent into S-shaped curves. Air could pass freely in and out of the flask, answering the objection that spontaneous generation required contact with air. But the curved neck caused airborne dust and microorganisms to settle in the bend before reaching the liquid. The broth remained clear and sterile indefinitely. When Pasteur broke the neck off a flask or tilted it so the trapped fluid in the bend flowed back into the broth, the liquid clouded within days as microbes grew.

He published the results in the Annales des Sciences Naturelles in 1861, and in December 1862 the Academie des Sciences awarded him the Alhumbert prize for the work. The experiments showed that microbial growth in the broth required microorganisms from outside, and that keeping those organisms out was enough to prevent putrefaction. The implication for medicine was direct: if microbes caused decomposition and, by extension, infection, then blocking their access to wounds or surgical fields should prevent sepsis.

In 1865 the Glasgow chemist Thomas Anderson pointed the surgeon Joseph Lister to Pasteur's work. Lister drew that conclusion and began applying carbolic acid to compound fractures at Glasgow Royal Infirmary the same year, publishing his results in 1867. Pasteur himself went on, in the 1870s and 1880s, to study anthrax and fowl cholera and to develop vaccines against fowl cholera, anthrax, swine erysipelas and rabies, in open rivalry with Robert Koch's German school.

Pouchet did not concede and went on defending spontaneous generation. Several of Pasteur's swan-neck flasks are displayed at the Pasteur Institute in Paris, and more than 150 years later the broth in them is still clear.