In Louis Pasteur’s swan-neck flask experiment, what entered the broth when the flask neck was broken?

In Louis Pasteur’s swan-neck flask experiment, airborne microorganisms entered the broth when the flask neck was broken.

Pasteur boiled nutrient broth in glass flasks with long, curved necks. Air could enter, but dust particles and microbes settled in the bend instead of reaching the broth. The sterilized broth remained clear for a long time, supporting the conclusion that microorganisms came from the environment rather than appearing spontaneously.

When Pasteur broke the neck or tilted a flask so the broth touched trapped dust, microbial growth appeared. This was a crucial challenge to spontaneous-generation claims and helped establish modern germ theory. The experiment did not show that air itself was alive; it showed that air could carry living contaminants.

Pasteur’s work followed earlier sterilization studies by scientists including Francesco Redi and John Tyndall. A common mix-up is thinking the experiment proved that all life comes directly from other life in every possible setting; its immediate conclusion concerned contamination of nutrient broth under the tested conditions.