Spontaneous generation

It was once believed that microbes generated spontaneously. For example, meat was believed to rot because microbes spontaneously formed from the blood. As spontaneous generation became increasingly controversial, the following experiments were performed:

These results support that airborne microbes were settling into the broth.

However, some people argued that growth did not occur in sealed flasks because there was not enough circulation. Pasteur proposed the U-Tube experiment, using a specialized flask he developed. Shown adjacent is a diagram of Pasteur's U-Tube flask, and below is the protocol.

1. The flask is partially filled with nutrient broth.

2. The broth is sterilized via boiling.

3. The flask is incubated for 72 hrs.

4. Result: no growth.

5. The flask is tipped so that the broth spills into the dip in the neck.

6. The flask is incubated for 72 hrs.

7. Result: growth observed.

This experiment works because no microbes can fall into the broth. The microbes are captured in the dip in the neck. When the flask is tipped, though, and the neck is washed with nutrient broth, the broth gets polluted. That is why growth is observed in the second part of the experiment.