The science experiment: design, methodology and results

Hi! This is Max's mom Dora. Max was overwhelmed yesterday trying to explain his project (which ended up being quite complicated) and he asked me to do it for him.

This is a picture of a bacterial culture taken from the web

Research question: For his science fair Max decided he wanted to know which household products are more efficient in killing household bacteria. The whole family contributed in narrowing down the project to address the question of "Which of the following substances kill bacteria taken from your teeth best: toothpaste, hydrogen peroxide or tap water?".

Methodology: On three consecutive days we took a sample from Max's teeth with a sterile swab. It was done first thing in the morning before he has put anything in his mouth. We transferred the samples to Petri dishes prepared with agar. Each day after breakfast we treated Max's teeth: first day he brushed, second day he rinsed with peroxide for 20 secs, and third day he rinsed with tap water for 20 secs. Each time after the treatment we took another sample of the same spot in Max's mouth (the front teeth) and transferred it to Petri dishes.

All Petri dishes were labeled with kind treatment, date and time. We stored them the the warmest and, as it happened, the darkest place in the house - on a shelf in the corridor close to the thermostat in the hope that the room temperature would be more stable. Here is a breakdown of the Petri dishes:
#1 - day 1, bacterial culture from teeth without treatment
#2 - day 1, bacterial culture from teeth after brushing with toothpaste
#3 - day 2, bacterial culture from teeth without treatment
#4 - day 2, bacterial culture from teeth after rinsing with hydrogen peroxide (20 sec)
#5 - day 3, bacterial culture from teeth without treatment
#6 - day 3, bacterial culture from teeth after rinsing with tap water (20 sec)

Hypothesis:
Max hypothesized that the hydrogen peroxide would give the best results followed by the toothpaste and tap water.

Results:
The record of the results is a bit sketchy (Max is still learning how to do that), but here are what surprised us:
On day 6 of the experiment samples #2, #4, and #6 had developed visible bacterial colonies (little yellowish specs). Sample #6 had also some mold. But none of the samples without treatment had any visible bacteria in them. The first signs of bacterial colonies in the untreated samples started to appear  around day 10.

The results suggested that any kind of treatment introduced more bacteria than it removed. Because the non-treated samples took 4 additional days to develop, Max and his parents were extremely surprised. Consequently we repeated the day #1 and #3 experiments with similar results: the difference between treated and untreated cultures were a few days in favor of the untreated ones.

Next steps:
1. We will repeat the experiment taking detailed notes. We are waiting for the new Petri dishes.
2. We talked to our dentist and he suggested using plaque colorant to see how Max's teeth look like before and after treatment. Down side: the colorant colors the plaque, not the bacteria. We will do it anyway.
3. We thought that maybe the surprising result came from the fact that the bacteria in the mouth might need body temperature to develop. Mommy is still trying to convince herself that in the name of science she can sacrifice her oven and make it a temporary home for the Petri dishes. The lowest I can put it on is 100 F, which is more or less body temperature.

Questions for the readers:
1. What are your interpretations of the results?
2. What are your suggestions for improving the experiment? Bear in mind that the improvements have to be achievable at home.

Thank you in advance!!!