Which experiment demonstrated that air-borne microbes can contaminate broth unless dust is trapped?

• A. Pasteur's swan-neck flask
• B. Needham's broth
• C. Redi's meat-in-jars
• D. Koch's postulates

Answer: (A)

Airborne microbes travel with dust in the air, so a barrier that lets air in but blocks dust can reveal whether the air itself is the source of contamination. Pasteur’s swan-neck flask demonstrates this clearly: the broth is boiled to sterilize, then kept in a flask with a long, curved neck. The curve traps dust and any microbes, preventing them from reaching the broth even though air can circulate into the flask. As long as the neck remains intact, the broth stays sterile. If the flask is tilted or the neck is broken, dust-laden air can reach the broth and microbes appear. This shows that contamination comes from microbes carried by air, not from spontaneous generation.

Other classic experiments touch on related ideas but aren’t about this barrier effect. Needham’s broth experiments suggested spontaneous generation under certain conditions, but flaws in his method allowed microbes to contaminate; Redi’s meat-in-jars argued against spontaneous generation by showing flies lay eggs on meat, not that air alone seeds microbes; Koch’s postulates outline methods to link a microbe to a disease, not the source of airborne contamination.