Pasteur used swan-neck flasks in his experiments to prove that:

• A. Air is required for the growth of all microorganisms.
• B. Microorganisms do not arise spontaneously but come from pre-existing life.
• C. Microorganisms can grow in nutrient broth without any external contamination.
• D. Boiling destroys all forms of life, including spores.

Answer: (B)

Verified step by step guidance

1. Understand the historical context: Before Pasteur's experiments, the theory of spontaneous generation suggested that microorganisms could arise spontaneously from non-living matter.
2. Recognize the design of Pasteur's swan-neck flask experiment: The flask allowed air to enter but prevented dust and microorganisms from reaching the nutrient broth due to the curved neck trapping particles.
3. Analyze the outcome: After boiling the broth to kill existing microorganisms, no microbial growth occurred in the broth unless the flask was tilted, allowing trapped particles to contact the broth.
4. Interpret the results: This demonstrated that microorganisms do not spontaneously generate but come from pre-existing microorganisms present in the environment.
5. Conclude that Pasteur's experiment disproved spontaneous generation and showed that contamination from external sources is necessary for microbial growth.