Science Buddies: "Ask an Expert"

Hi everyone
My name's Vighnesh and I'm doing a project for my regional science competition which I need to finish in 2 weeks. I'll be investigating whether using mouthwash and toothpaste together is more effective than using them separately. My current plan is to take swabs of the mouth before and after using mouthwash, toothpaste and then both of them together. Will taking the swab and then zig-zagging it over a petri dish produce colonies of bacteria that I can count and compare? If not, what methods will result in the bacteria growing in countable and clear colonies? Really appreciate any help I get.
Thanks!
Vighnesh

Hi Vighnesh and welcome to Scibuddies.

Your experimental plan will work ok but you may not get separate, countable bacterial colonies. A cotton swab will collect some bacteria but it depends on where you swab and for how long. Also, the transfer of bacteria from the swab to the agar is not quantitative--oral bacteria tend to be sticky.

Were you planning to do the swabs at a specific time during the day on different days? Some people brush their teeth right after eating but others don't, so when you do the swab is important. Also, I assume you were going to swab on different days and do it at least three times for each treatment. You cannot get accurate data from a single trial. How many people do you have to do the experiment? The more the better! You should compare the treatments for single individuals but also for a number of people--how many I don't know. It depends on how variable your results turn out to be.

I still haven't answered your original question about the colonies. Swabs are simple to use but have a lot of variability. Why don't you try the experiment on yourself first and see if you can get consistent results for at least three trials. If you can't repeat an experiment then you can't believe the results.

Do some reading online and see if there is another, more quantitative way of sampling mouth bacteria. Maybe just swishing a measured volume of water in the mouth for a certain length of time and then spreading a measured volume of this solution on nutrient agar would give more repeatable results.

Good luck!

Sybee