Final answer:
Louis Pasteur's swan-neck flask experiment was designed to disprove spontaneous generation by demonstrating that microbial contamination of food requires pre-existing microorganisms, supporting the germ theory of disease and the concept of biogenesis.
Step-by-step explanation:
The principle behind Louis Pasteur's swan-neck flask experiment was to disprove the theory of spontaneous generation and provide evidence for the germ theory of disease. Pasteur used swan-neck flasks containing broth to demonstrate that microorganisms in the air were responsible for the contamination of food and beverages, not an invisible "life force" as proponents of spontaneous generation believed. His experiment showed that when the broth was boiled and the flask's neck remained intact, no microbial growth occurred, confirming that life did not spontaneously arise from the broth. However, when the swan-neck was broken off, allowing microorganisms from the air to come into direct contact with the broth, contamination occurred. Thus, Pasteur effectively demonstrated that life comes from pre-existing life, a concept known as 'biogenesis'.