Virtual Experiment: Bacteria

To better understand how the process of science works, we are going to start by performing a virtual experiment to test how different variables affect the growth of bacteria. We will use the online simulation here:

https://biomanbio.com/HTML5GamesandLabs/SciMethodGames/bacterialabpage.html

One common way of studying bacteria is to grow them in small containers called “petrie dishes.” This is useful because it gives the researcher more control over the various components of the experiment. In the images below, the white patches represent bacterial colonies. Each colony grew from a single bacterium that was introduced to the petrie dish, and survived to reproduce. Larger patches represent more bacterial growth. You may also be able to differentiate between various types of bacteria and other organisms.

Let’s set up an experiment to see how the simulation works. The variable that we will manipulate is whether or not sterile conditions were used while the experiment was being set up.

	Dish #1	Dish #2
Temperature:	25° C	25° C
Amount of Bacteria A:	1 microliter	1 microliter
Amount of Bacteria B:	1 microliter	1 microliter
Duration of experiment:	3 days	3 days
Use Aseptic Technique?	No	Yes

One thing you may have noticed is that conditions are as identical as possible between the two dishes, except the variable that we want to investigate. (In this case, whether or not we are using an aseptic technique to minimize contamination from outside sources).

• What do you think would be an appropriate hypothesis for this experiment?

Now, let’s conduct the experiment and see what happened:

• In looking at our two dishes, what type of useful data would you suggest we collect?
• Explain in your own words the results of this experiment, and speculate about the reason for any differences you see between the two different treatments.

There is one more thing we want to consider: how do we know that the results we see are due to the manipulation we performed, and not just due to random chance? In order to understand the strength of our results, we would need to perform some statistical analysis on our data. We will not do that today – we’ll revisit this later in the semester. But it’s important to know that experimental results are stronger – we have more confidence in them – when there is a large enough sample size. Performing the experiment just once with the aseptic technique and once without does not produce convincing results. So, we will replicate our experiment – performing the exact same manipulation multiple times. In this case, let’s perform the experiment on 5 different dishes using the aseptic technique, and 5 different dishes without using the aseptic technique.

Treatment #1 (No aseptic technique)

Treatment #2 (Aseptic technique used)

Data Collection:

Now, let’s collect our data. We’ll do this by counting the number of each type of colony on each of the dishes.

Treatment:		Dish 1	Dish 2	Dish 3	Dish 4	Dish 5	Average
1) No Aseptic Technique	Bacteria Type A (off white)	25	16	17	21	19	19.6
	Bacteria Type B (yellow)	7	8	6	6	10	7.4
	Mold (black)	17	22	20	20	20	19.8
2)Aseptic Technique	Bacteria Type A (off white)	21	24	24	21	25	23
	Bacteria Type B (yellow)	20	18	18	19	19	19
	Mold (black)	0	0	0	0	0	0

Now, can you draw any conclusions from these results? Provide an explanation for they differences that you see between or what caused any observed differences between Treatment 1 and Treatment 2? Support your speculation with evidence from the data above.

Design Your Own Experiment

Now, you and your lab partner will use the online simulator to conduct two experiments of your own, with assistance from your lab instructor. You will investigate one or two dependent variables in each experiment, run the simulations, and analyze the results.

Experiment #1

• Describe the first experiment you intend to set up:
• What is the hypothesis for your experiment?
• What is the independent variable in your experiment? (What will you manipulate)?
• What is the dependent variable in your experiment? (What will you measure)?

Perform your experiment 5 times for each treatment, and include the counts of bacteria in this week’s worksheet.

• Describe the effect – if any – that you witnessed. Why do you think the results turned out the way they did?
• If you were going to do a second experiment, what would you do differently?

Experiment #2

• Describe the second experiment you intend to set up:
• What is the hypothesis for your experiment?
• What is the independent variable in your experiment? (What will you manipulate)?
• What is the dependent variable in your experiment? (What will you measure)?

Perform your experiment 5 times for each treatment, and include the counts of bacteria in this week’s worksheet.

• Describe the effect – if any – that you witnessed. Why do you think the results turned out the way they did?
• If you were going to do a second experiment, what would you do differently?

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This semester, many of us will conduct a simple experiment where we grow plants at different densities to see if there is any effect on growth. In order to better understand this process, we will also work through a series of virtual plant growth experiments together, all of which test a different variable. Start by downloading the “lab journal” worksheet below. You will need to keep track of this document over the course of several weeks, so make sure to put it somewhere you will be able to easily access it until it is submitted.

Virtual Experiment: Plant Growth, Week One

Experiment #1: Effect of Light on Plant Growth

This experiment will test the effects of different amounts of light on the growth of plants. Plant growth will vary based on the amount of light the plants receive. You will observe the growth of four different treatment levels of light per day: 4 hours, 12 hours, 24 hours, and 0 hours.

View the video below, completing the Week One activities as you watch:

COMPLETE THE WEEK ONE ACTIVITIES IN THE LAB JOURNAL

Next week, you will watch Part Two and observe the growth of your plants.

At Home Plant Growth Experiment

Your lab instructor will now provide the details about plant the seeds for your own experiment. From the lab supplies kit you received in the mail, you will need the following supplies:

• Peat pots
• Seed envelope
• Labels
• Potting soil

You will also need:

• A spoon (for scooping soil)
• A pencil
• A sharpie (for writing on labels)
• A 1 cup measuring cup
• Something to keep the pots on after they are planted. Some things that could work:
  • Plastic container
  • Glass baking tray
  • Plastic bag

If you do not yet have your supplies, don’t worry! You’ll be able to view the video again when you are ready to do your own planting.