Bacteria are invisible to humans except under a microscope. Which evidence did Pasteur use to show that bacteria were the cause of soured wine?

Pasteur used an experiment with broth and a swan neck glassware. He placed broth in a regular, open container and bacteria grew because the bacteria could easily get into the broth. The swan neck glassware is a glass container that has a slightly horizontal neck that made it impossible for the bacteria to enter the broth. To this day Pasteur's experiment is on display and the broth is still not contaminated with bacteria.