My science fair profect is going to be, “What brand of mouthwash kills the most bacteria?” I need some ideas on how to do the experiment. I am thinking of using the mouthwash, then rubbing a cotton swab in my mouth and rubbing it in a container and seeing how many bacteria colonies grow. But what can I do as a variable? Can you give me any other ideas?
Your variable would be the brand of mouthwash. This is going to be a difficult topic to do since things that should remain constant (in a normal experiment) are going to vary in this one. IOW the amount of bacteria in one persons mouth is going to be different then in anothers, and the amount in each persons mouth will vary from day to day.
Forget bacteria… too hard, leave that for the pre-med wannabes. Lots of secondary school teachers and administrators are easily impressed technophobes. Anything involving computers and snazzily presented is a sure first place blue ribbon. How varying amount of RAM impacts the responsiveness of MS Office programs and Photoshop is sure fire winner. Just remember that presentation is everything. All text and graphic needs to be done on printers and neastly pasted up with glue stick (NO TAPE!) use high quality real foamboard, not the cardboard stuff. Printed pages of digital pics detailing the experiment steps and liberal use of colorful charts and graphs is also a plus.
Make absolutely sure the layout conforms precisely to the structure detailed in the instructions for how they want the experiment steps formatted.
I agree with the previous poster about the project idea. Also, it seems a little…well, sterile 
and not very original. Isn’t there anything else you’re curious about? I know these science fairs have enough rules to choke any real science inquiry to death. But how about looking into a question that isn’t so easy to answer? Something that, while you might not win any prizes, could really light you. Like, why aren’t there any really big bugs? What is the rate limiting factor that keeps them small? Or how come no animals have wheels instead of legs? Or can you design a streetlamp that doesn’t cause light pollution? Your project might be making various models to test some ideas and then write up the process and the results. And the answer “I don’t know” is a perfectly good scientific answer.
Anyway, maybe you can try asking a question that doesn’t have a straight up, yes-or-no answer. It would be more fun, you’d enjoy it more and who cares if you don’t win—science should be more than that.
Oops, quick posters here. I meant I agreed with the FIRST posted response. The second one seems complex for the sake of complexity and too deep for me…
That’s cool, I did the same experiment in high school and got second place. I could have done better if I had been able to talk to a microbiologist.
Basically what you want to do is mix up some culturing media with a certain amount of mouthwash in it. Then inoculate the media, wait a day or so, and then see which one supports the most/least bacterial growth.
Before you do the actual experiment, you will have to do several trial runs to find the right amount of mouthwash to mix in the culturing media. You want to find a dilution that will let enough bacteria grow, but not so many that counting them becomes difficult. You also need to have an incubator and plan ahead to make sure it will hold all the plates you will use. If you make your own, you must take great pains to show that the temperature is consistent.
If you want to be really thorough make sure you have more than one plate for each type of mouthwash, and also to the same trial using bacteria from different people’s mouths.
I think that the OP’s idea is excellent, since it asks a scientific question, resolveable by experiment, which has relevance to daily life. However, I can also state unfortunately that such an experiment may well be against the rules of the school or school district sponsoring the science fair. By the time I was in eighth grade, for instance, the Cleveland board of education had banned all science fair experiments involving a living organism in any way and all experiments using any flammable or toxic chemical, as well as banning displays which contained any liquid whatsoever (including water!) or anything which plugged in to a power outlet. Your experiment would involve living organisms (both the bacteria themselves and the human volunteers whose mouths you swab) and a toxic chemical (the mouthwash), and you would probably want to have a couple of your actual Petri dishes on display at your booth, all of which would violate Cleveland’s rules. I don’t know what the rules are where you are, but make absolutely certain that you do know, and that your project falls within them.
A picture says 1,000 words…
UK High School science technician checking in…
I can’t comment on whether you’ll be allowed to do this as I’m not familiar with the US Health and Safety regulations, but this is what we do…
We do a similar experiment seeing what brand on toothpaste or disinfectant works best, this is what we do:
Step 1: The bacteria.
The easiest way to do this is use muddy water that’s been filtered through filter paper, then use a sterile inoculating loop to smear it across the nutrient agar in the petri dish.
Step 2: The anti-bacteria.
This is what we do with with disinfectant, I’m sure it would be the same with mouthwash. Use a hole punch to punch some holes in filter paper, then soak the circles that have been punched out in the mouthwash (you could vary the time you soak them for, just keep it constant for the different mouthwashes). Place the circles on the petri dish.
Step 3: Growing
Close the petri dishes by putting tape across two sides, DO NOT SEAL THE DISH COMPLETELY, this will encourage anaerobic bacteria which are not healthy. Make sure you grow all your samples for a constant length of time (say one week), then observe.
Step 4: Observing
There should be a white ‘paste’ of bacteria over most of the dish, with clear circles around the soaked circles; the one with the biggest clear area around it is the most effective anti-bacteria. You should seal the dishes airtight all around the edges now. Maybe you could stick the dishes on a scanner to record your observations?
Step 5: Destroying
You’ve now sealed your dishes, once you’ve observed them you should destroy them immediately. Do this by putting them in an autoclave for 20 minutes.
Any questions don’t hesitate to ask.
Tuco.