He disproved the spontaneous generation

The sense that life might spontaneously develop in the past has led people to view fermentation and decay as spontaneous events. The French scientist Georges-Louis Leclerc, count de Buffon, and the English naturalist and Roman Catholic monk John Turberville Needham continued to stir controversy throughout the 18th century. Both of these individuals agreed with the idea of spontaneous generation while Italian biologist and abbot Lazzaro Spallanzani believed that life could never spontaneously emerge from nonliving matter.

He had long recognized that the continued acceptance of spontaneous generation would undermine his germ theory. Pasteur used a straightforward experimental method to get right to the point. He demonstrated how to pasteurize beef broth by boiling it in a "swan-neck flask," a long, curved vessel with a neck that could catch dust and other contaminants before they entered the body of the container. The soup eventually grew hazy and showed signs of contamination if the neck of the pot was left uncovered after the broth had been boiled. This experiment disproved the spontaneous generation theory since the boiling broth remained sterile even if it was never exposed to air. This not only resolved the philosophical conundrum surrounding the beginning of life at the time but also established a strong basis for the emerging field of bacteriology, which relies on aseptic handling and sterilizing methods.