Science Buddies: "Ask an Expert"

Hi
My daughter is doing a science fair project on the effectiveness of different types of mouth wash. We are using a wide variety of mouth washes. We are also exploring a homemade clove oil and charcoal mouth wash. She's testing the mouth wash on me. Every evening around the same time,2 hours after dinner we test a different mouth wash. then she swabs my mouth and applies it to a petri dish and we watch the growth. We do a different mouth wash each day. Its hard to quantify the results since there are a lot I tiny specs of bacteria growing. Its hard to count it all. We're taking plenty of pictures. Also since tests are being done daily the results seem off because everything grows at a different rate. Meaning we are comparing results from day 1 of a certain mouth wash against day 2 of another one. How can we execute this experiment better?

Hi Salma,
I think the experiment design is pretty good actually.
In order to keep all variables exactly the same (nature of food, exact time after meal, growth temperature etc.) one may think it's best to do it all on the same day. However, obviously that creates a problem as if you use the mouthwashes sequentially on the same day, they are not working on identical conditions and each subsequent mouthwash will encounter less bacteria in the mouth.
The steps you can take to reduce day to day variability and thus presumably only detect effects of the mouthwash are:
1. Maintain the same diet all 4-5 days over which the testing will be done. Mouth bacteria can be influenced by diet.
2. Make sure to keep other aspects of your oral hygiene identical on all the days (including toothpaste, brushing schedule and duration)
3. Definitely eat the same food each day in the meal prior to swabbing
4. Wait the exact same amount of time post eating, before collecting samples (2 hours is very reasonable)

The fact that the same person is being tested everyday is a very good way of reducing variability as oral microflora differ person to person.

Re: " Meaning we are comparing results from day 1 of a certain mouth wash against day 2 of another one."
I think this is not necessarily the case although I understand your concern. For the sake of simplicity, let's assume that each day you are looking at the immediate effect of the mouthwash used on that day. It is really difficult to take into account the effect from the previous day's mouthwash and it is unlikely to persist after a full day of normal meals.

Re: "everything grows at a different rate." - maybe try growing the plates at the same temperature (preferably 37 deg C) and same amount of time for each sample. Let's say in one plate, you see colonies in 12 hours and in another, only after 24 hours. For consistency, pick a time (say 24 hours) and for each, ask how many colonies were obtained in 24 hours. That way, at least one variable (time) is constant and under your control.

Hope this helps! Please write back if you have more questions.
Best,
MS