Science Buddies: "Ask an Expert"

My kindergarten students are growing bacteria in petri dishes with agar. They are swabbing inside their mouths with a cotton swab and putting it on the agar. I know I need a control but I am not sure what I do. What would I use for a control? Also, does the temperature have to be up to 90? How long should it take before my kids see some results!
Thanks!
Mary

Hi Mary,

A control for bacterial cultures would include a positive-growth plate with known organisms that you expected to grow to verify that your plates provided a suitable medium for growth, and an uninoculated plate that should show no growth. This is more of a demonstration than an actual experiment, but I suppose you are answering the question, "what is growing inside of our mouths?"

You can incubate the plates anywhere from ambient to 99 degrees F/37 degrees C. Most mouth bacteria grow best at body temperature, but many will grow at lower temperatures, although the growth rate will be slower.

If you do this experiment again, you might try using a soil sample, as this type of sample will contain more brightly colored organisms. Soil bacteria are exposed to UV light, so pigmented organisms have a survival advantage.

Be sure and tape all of the Petri dishes closed so the children can't open the plates after they grow. Mouth bacteria could potentially be pathogenic. You can soak the plates in a bucket of dilute bleach after you finish the experiment.

Donna Hardy

Donna, thank you for your help. However, how do I obtain a "known organism" ?
Thanks! Mary

Hi Mary!
In this case, based on your experiment, these "known" organisms would include bacteria commonly found in the human mouth. For more information on these types of bacteria, here is a website which may be of use-
http://www.textbookofbacteriology.net/normalflora.html
I hope this helps, best of luck to your students!

Hi Mary,

Your samples can actually be considered the positive "growth" control, since you inoculated a mixture of organisms. Occasionally, when doing a science fair project, the agar in the Petri dishes will dry out and there will be no growth on any of the plates. If this happens to you, the assumption should be that there was something wrong with the growth medium, not that there were no bacteria present. So if you get growth in the plates, that will be your positive control. If you incubate one plate without inoculating it and there is no growth, then you can conclude that the bacteria growing on the other plates came from the students' mouths.

Don't worry too much about explaining the concept of controls; just make sure the kids enjoy learning about science. However, if you are going to turn this in as a class project in a science fair, do let us know if you need more help.

Donna Hardy

Donna, I appreciate your reply concerning what I would use as a control. You suggested swabbing the mouth; however, that won't work because swabbing the children's mouths and noses was in our project.
You suggested using a "known organism" but where can I find one other than inside the mouth and what would the name of this organism be?
Thanks!
Mary

Hi Mary,

You could use a small amount of spoiled milk, a spice like pepper or dehydrated onion, or a couple of drops of water from a rain puddle that has been standing for a few days. All of these are known to contain bacteria and would grow a variety of colonies if transferred to a Petri dish with nutrient agar, however identification would be beyond the scope of your experiment. If you know someone who works in a microbiology lab, you could ask for a sample of E. coli (the recombinant kind, not the pathogenic variety) or a Bacillus species to use. The purpose of the positive control is to show that your medium will support the growth of bacteria, so if you do get growth on the samples from the children, that, empirically, would be a positive control. If your hypothesis was something like, "I think everyone has Staphylococcus aureus in their mouth and noses," then you would need a culture of the specific organism to grow and compare to. But your experiment is designed to demonstrate that bacteria do exist in mouths and noses, so any growth from the samples would be a positive control. The uninoculated, negative control plate is actually all you need for this experiment. I really don't think you need a specifically identified organism for your experiment. Your results are "growth"/"no growth."

I hope I'm helping here. If you think am not understanding your concern, then let me know what question you are trying to answer, and what your hypothesis is, and I'll try again.

Donna Hardy

All you want is a control to show that what is growing on your plate is due to whatever you swabbed and not due to something in the air. So a control would simply be an empty plate incubated along with the other plates. Or simply a plate that you streaked with a clean swab. This also checks that your swabs are sterile.