Science Buddies: "Ask an Expert"

Also, I left the plates under the different types of lights for about 2 hours and then gathered them together and put them under the incandescent light for it to grow...

The growth of bread yeast may be inhibited by light, although when you tested the dilutions they grew ok under incandescent light.

You did not tell me how close you had the different kinds of light to the plates. Also, how close were the plates to the incandescent light? Did you put the plates near the light in order to keep them warm?

Next time send me your experiment procedure and I will check it first. I would have advised wrapping the agar plates in aluminum foil to keep out the light. The warmth from the incandescent light helps the yeast to grow, but the bright light may inhibit them.

Alcohol on the spreader would have killed both mold and yeast, so that could not be the explanation. Maybe black mold is a lot less sensitive to light than bread yeast, so the mold spores in the air that drifted onto the plate while you had the lid off spreading the yeast might have grown in place of the yeast colonies.

I wish I had a better answer for you, but I really can't explain your results other than what I have already said which is mostly speculation based on reading what other scientists have published about yeast.

You did learn a lot about working with microbes, making dilutions and spreading agar plates--all useful skills for future science fairs.

Good luck!

Sybee

All the plates under all the lights are 16.5 centimeters away from the desk lamp... After the plates were under the different types of lights for 2 hours, it was all put under the incandescent lamp also 16.5 centimeters away from the desk lamp...

How did you explain the presence of black mold in place of yeast?

I said there could be mold spores in the air while I was opening up the lid to spread the yeast, or maybe since I left the plates out in room temperature for about 5-6 days there were condensation which could have created mold because it was moist...

True, there are mold spores in the air that will grow on nutrient agar, but why did your dilution test plate show only yeast colonies while the plates you put under the lights had only mold? Do you think the light killed the yeast but left the mold to grow? Does your biology teacher have any ideas about what caused your results?

I wish I had a better answer for you. For your next science project, make sure you have enough of whatever you need to repeat the experiments--a couple of times, if necessary.

Good luck!

Sybee