How did Louis Pasteur disprove spontaneous generation?

To disprove the theory of spontaneous generation—the now-discredited notion that life arises from nonliving matter—Louis Pasteur conducted a series of masterful experiments in the mid-19th century.12

Pasteur utilized specialized "swan-neck" flasks to test his hypothesis. He boiled beef broth within these flasks to sterilize it. The unique, long, bending neck of the flask allowed air to reach the broth but trapped dust particles and other potential microbial contaminants in the curve of the neck.1

As long as the broth remained isolated from these trapped contaminants, it stayed sterile, proving that life did not spontaneously generate from the broth itself. However, when Pasteur broke the neck of the flask, exposing the sterile broth directly to the air, the liquid eventually became cloudy due to microbial contamination. These results demonstrated that biogenesis—the principle that only preexisting microbes can give rise to other microbes—was correct. This work provided the foundation for modern bacteriology and validated his germ theory.12