IV. Experiment

Brand and type of food (bread).
Area (cm^2) of the food (bread).
The amount of surface area (cm^2) the food (bread) is in contact with the tiles.
Distance (cm) from phone camera to petri dish when taking the photograph.
Mass of agar in the Petri dishes.
The temperature the agar plates are stored in (37 °C).

Diagrams

Procedures

Preparation:

1. Set up the equipment as shown in Fig 2.1.1 (experiment setup diagram)

2. Label 6 agar plates and 5 plastic trays (excluding the control experiment) according to the 6 independent variables.

3. Using the cleaning solution (premade), wipe the table counter and equipment clean.

4. Take two slices of bread and cut them both into three equal rectangles with a pair of scissors, as shown in Fig 2.2

Fig 2.2 (Cutting of bread)

5. Place one piece of bread onto one plastic tray (the other two rectangles of bread are for the repeated experiments).

6. Repeat steps 4 & 5 for all five slices of bread.

Dirt solution:

7. Take a bowl and add in 1 banana, two eggs and a cupful of soil, and mix the contents in the bowl with a spoon thoroughly. This is to make the dirt solution.

8. Smear the dirt solution onto the table counter evenly.

Experiments:

Control (no bread) :

9. Take the agar plate labelled “control” and close the lid tightly to prevent bacteria and other microbes from contaminating it. Do not touch the inner surface of the agar.

0 seconds (bread not dropped) :

10. Take a cotton swab and rub it onto the middle of the slice of bread (as shown in Fig 2.3) that was on the plastic tray labelled “0 seconds”.

Fig 2.3 (Swabbing of bread)

11. Taking that same cotton swab, rub it evenly along the agar plate with the matching label, “0 seconds,” and close the lid tightly. Place the bread back onto the plastic tray.

Rest of the experiments:

12. Using a stopwatch to measure the time, take the piece of bread on the paper plate and drop it onto the dirty surface for 5 seconds, as shown in the example in Fig 2.4. (Do not drop another piece of bread over the same area that had already been used in the previous experiments).

Fig 2.4 (Timing the time the bread is in contact with the floor)

13. After the time is up, pick it up immediately and rub a new cotton swab onto the middle of the bread.

14. Taking the same cotton swab, rub it onto the respective agar plate and close the lid tightly. Place the dropped bread back onto the paper plate.

15. Repeat steps 12 to 14 for the other three pieces of bread with different timings.

Incubator:

16. Turn the incubator on and set it (heat it up) to 37 °C.

17. Place the six labelled agar plates into the incubator upside down. This is to prevent any evaporation from the agar from happening as the temperature increases.

Collection of data:

18. Take a clear plastic sheet and draw out a 1.0 cm by 1.0 cm grid with a marker and ruler. Ensure that there are enough grids drawn to cover the cover of the agar plate.

19. Let the six agar plates rest in the incubator over a timespan of 3 days; take a close-up top view image of each agar plate each day with the transparent sheet done in step 18 on top of it with a smartphone as shown in Fig 2.5.

Fig 2.5 (Taking pictures of the results)

20. Repeat the “Preparation” and “Experiments” steps two more times, using the same dirt solution, and remaining bread from the first experiment.

Data Analysis

21. Count the number of grids with bacteria to find the total number of grids of bacteria in that specific agar plate.

22. Record the results in the tables (Annex 2). Summarise the results into another table (Annex 1).

23. Calculate to find the average readings, and plot a bar graph of the number of grids of bacteria recorded against the number of seconds spent in contact with the floor.

24. Repeat steps 21 to 23 for all the agar plates and record the readings with their respective variables. (Day 1, a) (Day 2, b) (Day3, c)

Empty Table

Hypothetical graph

Risk Assessment

Pictures

Here are some pictures of us doing the experiments.

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