Experiment 1: Mold Growth on Bread and Fruit--What are Optimal Conditions for Fungal Growth?

Fungi are unable to ingest food. Instead, they must absorb nutrients (like bacteria) rather than ingesting them (as is the case for animals). Fungi secrete enzymes into their surroundings to help breakdown their food source and facilitate the absorption process. Fungi frequently have different growth requirements than bacteria. This can sometimes enable them to grow in places bacteria cannot. For example, fungi can grow at a pH of 5, which is too acidic for most bacteria. Some fungi can also tolerate higher osmotic pressures than bacteria can. This enables them to grow in areas with higher solute concentrations. Further, some fungi can use carbohydrate sources (such as those from wood) for energy that most bacteria cannot use. In this experiment, you will determine which factors encourage mold (filamentous fungi) growth on pieces of bread and apples.

Procedure

1. Add 120 mL of water to a saucepan or large microwaveable bowl and mix in ½ cup of sugar.
2. Gently heat the water/sugar mixture on the stove or in the microwave, stirring occasionally until all of the sugar is dissolved. Use caution when removing the solution from the microwave/stove as it will be hot. Allow the solution to cool.
3. Label the plastic bags with these titles: “bright, control”, “bright, water”, “bright, sugar”, “bright, lemon”, “bright, vinegar”, “dark, control”, “dark, water”, “dark, sugar”, “dark, lemon”, and “dark, vinegar”.
4. Cut the two slices of bread into 10 total pieces (5 pieces from each slice).
5. Cut the apple into 10 slices.
6. Place one piece of bread and one slice of apple into a plastic bag. Repeat this process until you have 10 bags, each with one piece of bread and one slice of apple.
7. Measure 120 mL of plain water in a clean, separate bowl.
8. Add 1 tsp. plain water to the bags labeled as water; add 1 tsp. of the sugar-water solution (prepared in Step 1) to the bags labeled sugar; add 1 tsp. of lemon juice to the bags labeled lemon; add 1 teaspoon white vinegar to the bags labeled vinegar. Do not add anything to the bags labeled control.
9. Find a well-lit, warm location in which to place the bags labeled bright (for example a windowsill or under a lamp).
10. Find a dark, warm location in which to place the bags labeled dark (for example a warm closet).
11. Examine the bags every 1 - 2 days for 2 weeks and look for mold formation on the bread and apple pieces. Take particular note to which environmental condition(s) mold formed in first and last. Also, make sure to note which environmental condition(s) grew the most and least mold. Use the following scale to assess the extent of mold growth:

    0 = none

    1 = little/some (1-25%)

    2 = moderate (26-50%)

    3 = heavy (51-75%)

    4 = extensive (76-100%)

12. Record your results in Table 1.
13. Do not open the bags when you are finished with the experiment. This will disperse mold spores and contaminate your work/living area. Dispose of the sealed bags in the trash and remove the trash bag from your living area.