Help with collecting and studying bacteria

[mariG123]

I am helping my daughter (4th grade) with her science fair.
The idea is what is the best nighttime beverage for your teeth water vs juice (what will show most bacteria growth). 1. Can she do the test by having friends come over, brush and then 'swish' with juice and do a swab right then? Would just doing a series of brushing and swabbing show really how much the juice vs water etc. will develop bacteria on the teeth overnight? Does she even have to wait an hour or so before collecting a sample? OR should she really have them brush at night, swish with juice and then test in the AM? (She would have to show them how to do it or show up very early w/the petri dishes. Would there be a problem in transporting the dishes from our house to other people's houses and home to study and HOW SOON should she be looking for noticeable growth? She already understands about having a control etc. so does not need help on that. Thanks!

[donnahardy2]

Hi Mari,

Welcome to Science Buddies! This is a really great idea for a science project. Your questions are good ones. Bacteria are present in the mouth at all times so the data points could be collected at any time. If you are measuring before and after samples, it takes about 20 minutes for bacteria to double in number, so waiting at least an hour after drinking the beverage could give measurable results.

I recommend doing a small pilot experiment and measure bacteria at time zero, one hour, two hours, and after overnight to see what happens before doing an experiment involving friends. Also, definitely have your daughter personally supervise data collection to help keep experimental conditions controlled as much as possible. It would be helpful to have a detailed written protocol before doing the pilot experiment.

After the sample is transferred to an agar plate, the bacteria will grow overnight if the plates are incubated at 37 degrees Centigrade (body temperature), or within 2-3 days if incubated at 20-22 degrees C (room temperature).

One precaution before continuing. Has your daughter discussed this project with her teacher? Rules for doing science projects involving growing unknown bacteria have changed recently and local rules vary, so you do need approval before doing the experiment. Here are the rules for doing projects involving potentially hazardous biological agents. I think that if the agar plates are sealed and placed in plastic bags before incubation and never opened, that it would be acceptable, but definitely do check with the teacher.

http://www.sciencebuddies.org/science-f ... ents.shtml

And, there are more rules for doing projects with humans. You will need to get a signed consent form from the parent or all of your daughter’s friends before doing the experiment.

http://www.sciencebuddies.org/science-f ... ects.shtml

Good luck. Please post again if you have any more questions.


Donna Hardy

[mariG123]

Hi--The science fair may not allow her to grow bacteria outside of a professional lab (new rule issue this year). WOULD test the PH levels after a night of brushing and going to bed, then a night of brushing, drinking water and going to bed and finally, brushing, drinking juice and then going to bed show a 'bad PH level" after drinking the juice? I mean would it show an increase in bacteria growing environment or would it NOT show in the salivia that way? Would real change in salivia have to do w/overall nutrition etc.

Is there anything that the ph level of saliva could show the next morning? IF she can't do bacteria growth....

Thanks!

[donnahardy2]

Hi Mari,

It was good that you checked before you started growing the bacteria.

Your idea of using pH measurements to indirectly measure bacterial growth is a good idea. Normally the pH of saliva is 6 to 7.4 and bacteria growing on a sugar source in the mouth should lower the pH. And it's not the bacteria that cause the problem with teeth; it's the lowering of the pH caused by the fermentation of sugar to acid.

http://en.wikipedia.org/wiki/Dental_caries

One problem you might encounter is that saliva has a natural buffering capacity and eating will stimulate the release of bicarbonate ions that will neutralize the pH. So, it might be challenging to measure.

Here's a paper that reports a method of measuring pH using a pH electrode built into a dental brace. The authors were doing a flavor study, but the pH measurement details of the experimental protocol and the results are interesting. You would not be able to reproduce the method, but using pH paper could make a good substitute. Please note that the authors found that a normal pH was restored in the mouth 250 seconds after eating.

You should do a small trial to make sure that you can measure a difference in pH in your test subjects.

http://pubs.acs.org/doi/pdf/10.1021/jf9806558

Here is information from the Science Buddies website on measuring pH:

http://www.sciencebuddies.org/science-f ... cale.shtml

Another possibility for measuring results for your daughter's project would be to use a vegetable dye to stain plaque on teeth, You would need a camera to record results, but the results would be visual, which would make a good display on the science board.

http://www.dental-health-advice.com/dis ... blets.html

Please post again if you have questions, or if there is any problems with proceeding with the pH or the vegetable dye project. We'll think of something else.

Donna Hardy