Science Buddies: "Ask an Expert"

hello-
my daughter is in 4th grade and is doing her first science fair project - Who's mouth is cleaner a dogs or a humans. She started the experiment and swabbed the dogs mouths and then swabbed the Petri dishes and we've been trying to incubate them but we're having some troubles. Our house is only about 70 degrees so I'm thinking it's not exactly the best environment for the bacteria to grow because most of the dishes have no growth on them and its been 48 hours now. We were incubating them in a cardboard box in a dark place but with no real heat source. We then turned a desk lamp on it to try and heat it up some but were thinking we need to make some type of incubator to keep them at a consistent temp bc it doesn't feel safe keeping a desk lamp on a cardboard box 24hrs a day. Does anyone have any advice on a cheaper diy incubator or links that we could do some research that would be at her age range to help her? Thanks so much. Also, since some of them aren't growing anything it could have been user error - should we have used some sterile water and wet the swab before swabbing the dogs and humans mouths? Any other tips and advice would be greatly appreciated. Thanks so much!

Hi, regards your specific question, here are some links for DIY incubator:
https://www.youtube.com/watch?v=LEsv0Qvbczs&t=119s

This link has other suggestions as well
https://labsup.net/blogs/blog/how-to-ma ... iI_f2zA_aL

Another thing to try is: Have your child ask their school to see if they have an incubator you could use

Wetting a swab with a sterile saline solution before swabbing your mouth helps increase the collection and transfer of bacteria, but it must be used immediately to prevent contamination. This is a helpful link: https://www.youtube.com/watch?v=A0b6_kg2oMc. It might be good to review the technique and check the growth medium in the petri dish you are using.

Another thing to remember is that it can take as long as a week to see visible colonies depending on how cold the room is. Wait for 5-7 days before writing off the plates as failed attempt. Also remember to have a plate with no swabbing as a negative control. It can help with such scenarios.

I want to add one quick caution. Please confirm that your student's teacher or science fair allows plates to be incubated at home.

Many fairs don't, because once bacteria are grown on a plate, the risk level increases. At minimum, you may find rules that require sealed-plate handling only.

Especially given your student's grade, it is important to check these rules so the project stays safe and eligible.

Amy
Science Buddies

thanks so much for the reply with the links and tips, Amy. I went out and got a small fish tank and a small reptile heater and surrounded the fish tank with cardboard. The dishes that were put in there still didn't seem to have any growth like I would have expected a mouth to have.There is also a lot of condensation in the plates and I'm wondering if that could be effecting the growth. The minimal growth that is there looks like water droplets on the agar so I'm not sure if its actual growth or from the condensation. The incubator wasn't getting above 78-83 degrees so I have moved it to a different location in my house thinking maybe there was a draft or something because it was closer to a door. the temp probe is now reading 90 degrees so I'm going to get some more plates and have her try reswabbing and see if we get better results.

we did have to turn in the idea proposal in to her teacher that we were going to be testing to see who has a dirtier mouth dogs or humans and there was nothing said about not being able to plate/grow anything.. but I will talk to her on Monday and make sure. I did tape all of the plates right after they were swabbed and then will put them into ziplock bags.

On another note - would you have any ideas on what the best way to chart or graph the growth/colonies on the plates would be? Should I have her count the colonies on say days 3,5,7 and then make a graph comparing those? or would there be a better way to go about this? Thanks so much for all of your help.

Hi
That's great that you are getting more plates!

For the data-presentation, your idea to show the colonies per plate by day for human vs dog, sounds great, using a different color or symbol for each. Did she make several plates for each? Using a separate swab for each plate?

Showing the colonies for each plate for human vs dog would give a good display of the variability within each species!

Also, photos of the plates by day would make a nice display for the judges at the fair.

One more detail ... can you check if the temperature is about the same around the tank? As they say, details details details!

Let us know how the tank works out!

Madeline

Another thought .. you might try putting the tank in a closet and see if the temperature gets closer to 95 F (35 C which is a common temp setting for incubators).

And, yet another detail to keep track of ... record the temp, perhaps morning and evening ... so she can show the judges how constant that is!

Madeline

MadelineB wrote: ↑Fri Dec 05, 2025 4:42 pm Hi
That's great that you are getting more plates!

For the data-presentation, your idea to show the colonies per plate by day for human vs dog, sounds great, using a different color or symbol for each. Did she make several plates for each? Using a separate swab for each plate?

Showing the colonies for each plate for human vs dog would give a good display of the variability within each species!

Also, photos of the plates by day would make a nice display for the judges at the fair.

One more detail ... can you check if the temperature is about the same around the tank? As they say, details details details!

Let us know how the tank works out!

Madeline

Thanks Madeline thank so much for your reply and help!-

So she swabbed 4 different humans with 4 different swabs and placed those on 4 different Petri dishes and then she swabbed 4 different dogs with 4 different swabs and placed those are 4 different Petri dishes. So we have a total of 8 Petri dishes that we're incubating. She has been pretty good about taking pictures. She took pictures of the initial swab then on day 1,3,5 and 7. We're still working on how to get the best pictures since the dishes are taped shut and there are glares on the dishes and sometimes hard to see the growth.

I just checked the incubator and the temp is up to 96.9 degrees. so I think it was having trouble keeping the heat in and the cardboard I taped on is helping and I think it also helped to move it away from the door as we have snow here now so I'm sure its cooler in that area. I never thought to record the temp to show the consistency. I doubt that the temp around the tank is about the same bc our house is only around 72 degrees.

I have a follow up question -

How would she go about counting colonies on a plate that have so much growth and the colonies are so small and close together it's hard to count all the little dots. Is there a method that would make it simpler for her or a way to estimate the colonies on a plate? or would that not be good for the results?

For the counts with heavily grown plates ... she could record that count for that plate as TNC for too numerous to count. On the graph, I'd put "TNC" at the extreme top of the vertical axis.

With 4 humans and 4 dogs, I would use 2 colors, say red for humans, and green for dogs, and a different symbol for each subject, so you can follow the increase/decrease in counts for each subject.

If all the counts for a "subject" (human or dog) plot the color/symbol
at the top. Hopefully the day 3 counts are "countable"?

If she is repeating this with new plates, maybe record counts for day 2 which might be countable even if day 3 is TNC.

Interesting ... as you've seen, the colonies grow slower at lower temperatures, so maybe 85 - 90 F would allow for enough growth to see the increase in counts, without having most of the plates show TNC. Can you set the temp on the reptile heater?

I think the judges will be interested in reading about these details (don't think of them as insurmountable problems) in her project presentation! Good work!!

MadelineB wrote: ↑Fri Dec 05, 2025 5:35 pm For the counts with heavily grown plates ... she could record that count for that plate as TNC for too numerous to count. On the graph, I'd put "TNC" at the extreme top of the vertical axis.

With 4 humans and 4 dogs, I would use 2 colors, say red for humans, and green for dogs, and a different symbol for each subject, so you can follow the increase/decrease in counts for each subject.

If all the counts for a "subject" (human or dog) plot the color/symbol
at the top. Hopefully the day 3 counts are "countable"?

If she is repeating this with new plates, maybe record counts for day 2 which might be countable even if day 3 is TNC.

Interesting ... as you've seen, the colonies grow slower at lower temperatures, so maybe 85 - 90 F would allow for enough growth to see the increase in counts, without having most of the plates show TNC. Can you set the temp on the reptile heater?

I think the judges will be interested in reading about these details (don't think of them as insurmountable problems) in her project presentation! Good work!!

Thanks Madeline

I like the idea of recording TNC for the large amounts. because the growths on the pate are so tiny they're almost impossible for my daughter to count correctly. I'll try to post some pictures to see if you think that they're even growing properly.

She did redo the plates but I feel like nothing is really growing well on them and I'm not sure if it's the agar that is faulty because they're being shipped in such cold weather? but the growths are so different looking from the first time we did it. She did check the plates daily and even from day 1 to day 2 the amount of growth that showed up was way too much for her to count correctly. will the results look bad if for 3 of the dogs by day 2 she has TNC for the plates?

There unfortunately isn't a way to set the temp on the reptile heat mat. we have it taped to the side of the fish tank bc we were worried that it would make the bottom of the tank too warm where the plates were sitting. Is there anything else that we could do to make the experiment better? or for a 4th grader is she doing okay?

Also, is there anywhere on the website that we could look up some possible questions that the judges might ask her during her presentation? This is her first science fair and very shy so she is extremely nervous for the presentation portion.

Hello Lindsey,

Thank you for your careful observations and questions! Here's my comments ... but I hope other Science Buddies experts chime in with their ideas too!

First, remember the judges understand that not all projects work out perfectly and lots of valuable information can be learned from every experiment!

It would be good to have a data table/log for each of the two sets of plates, with a description of what she sees for each subject on each day. Judges will be very interested in this information!

They will also like to see a "trouble-shooting" section with her ideas for what went wrong and what she changed for each set of plates.

One question ... since it sounds like the plates for the dogs are the ones that are TNC, were those plates closer to the heat source?

Second, should she make it better? Carefully writing up what she has for the two sets of plates is great for a 4th grade project ... if she does want to do a third experiment with more plates ... here's some suggestions for using a lamp as a heat source ... But she could consider not doing the 3rd experiment, but describing what she will try for next year!

viewtopic.php?t=7673

viewtopic.php?t=7571


Third, for preparing to talk with the judges, here's some suggestions:

https://www.sciencebuddies.org/science- ... 0interview

This looks like a long "to do" list ... but carefully writing up her methods and observations for what she's already done is what is important!

Let us know if you have more questions and best of luck talking with the judges!

Madeline

I forgot to attach an image of her Petri dishes. do these look normal or does the growth look "off" I feel like on all of the plates they are growing "clear" colonies. Also, could these plates be considered TNC since the dots are so tiny and she can't really see some of them?

I never thought of a trouble shooting section. that's a good idea! would that be just something that she talks to the judges about or is it something that she should put in her report or lab notebook? we have a lot that we would do differently next time lol

on the new plates that were testing it seems like the adults and children are the TNC and I have all of the plates pushed to the far wall of the fish tank.

Hello Lindsey,

Thank you for the very clear photos! I'm curious to see photos of the dog plates?

I will answer some of your new questions now, but it will take me a bit longer to respond about the counting ...

I suggest that she definitely include a specific section in her project display describing trouble-shooting and what she'd do differently.

I also suggest that she clearly describe that the two separate setups/results with the first being results before you used the tank and reptile heater and the second with the tank and heater. Describing the trouble-shooting between these 2 setups/results is important ... carefully documenting what works and does not work and what is done to improve is what the scientific method is all about! This is excellent for her first project!

Here's a link to an outline for her report ...
https://www.sciencebuddies.org/science- ... nal-report

and arranging her report on the display ...
https://www.sciencebuddies.org/science- ... lay-boards

This outline makes it look like the science project works the first time ... but life is not like that and it's important to show how she learned from the first setup/results!

I'll be back with some comments about counting ...

thanks for your patience!

Madeline









https://www.sciencebuddies.org/science- ... nal-report

Hi Lindsey,

Some suggestions about "counting" ...

First, in "statistics", there are two types of variables ... quantitative and qualitative. "counting" the number of colonies is quantitative. Another quantitative variable would be recording size of the dogs based on weighing each dog.

One simple example of a qualitative variable would be classifying the "size" of the dogs as small, medium, and large.

She could create a simple qualitative variable for the colonies on the plate in several ways. One way could be to score each plate at each day based how "full" of colonies each one is, like

empty
some colonies
about half
mostly covered

I will write yet another post to talk about how to display results using such a qualitative variable!

Second, another thing to consider when discussing what might influence the results ...

Are all the plates at a specific day for the dogs "similar"? Or are the results for 1 or 2 dogs very different from the other dogs? Here it might be easier to use the qualitative measure for deciding "similar"

And ditto for the human subjects ...

In these two experiments, the initial amount of colonies may vary for each subject ("day 0") and has not been measured. The amount might vary depending on time since the subject ate or drank, or what they ate. She could describe how to deal with these possibilities in a future study!

Or might the "size" of the dog or size of mouth affect how many colonies were in the initial swab?

Let me know if any of this helps to make sense of the results!

Madeline

here are some of the plates with the dogs swabs.

more dogs

Hello Lindsey,

Thanks for posting the photos of the the plates for the dogs!

Looking back thru this thread, I see that I've offered different suggestions for "counting" the colonies. Now, having seen the plates for both the humans (?Day 1) and dogs (Day 3, 11/23), I now think the most informative display of the results is to display the actual plates ... by individual dog by day (1, 3, 5, ??7) and by individual human by day and separately for the two different sets of plates.

It sounds like she already has the photos ... so instead of taking photos again, she could arrange the existing photos in a table-like display with dogs (humans) being the rows and day being columns in the table, with a table for the first results and a separate table for the second set of results.

Be sure to carefully describe the incubator setup (with changes over time?) for each setup, and show the temperatures she recorded by day (time?).

She could print the plates and cut and paste them into each table. She could then take a photo of each table and show that in her project display for the judges.

IMHO the key features of this are (a) careful descriptions of the setups, temperatures, etc and (b) discussion of all the possible explanations for the differences/similarities in results both between dogs, between humans, and dogs vs humans.

She might want to include the rationale for showing the actual plates instead of showing quantitative or qualitative counts, in case some of the judges are skeptical that showing the plates is not "statistics" ... In fact, the plates are statistics. Statisticians use two useful terms ... the first is "statistical information" and the second is "sufficient statistics". From a statistical point of view, the actual plates are sufficient statistics and contain more statistical information than qualitative summaries would, and certainly more information than reporting "TNC"!

Let me know if you or she have questions about this new approach!

Good luck!
Madeline

A couple more thoughts ...

If she has time, for extra bonus credit, she could use the internet (google, duckduckgo, etc) to find results of others who have done this experiment and report their results ...

And, just for future information, in the Science Buddies Reference Materials, this link: https://www.sciencebuddies.org/science- ... leshooting, has this link to "make your own incubator":

https://www.sciencebuddies.org/cdn/scie ... ubator.pdf

It suggests using a lamp for a heat source with a dimmer to have more control of the temperature!

And, in an earlier post in this thread, the SB expert "aredlife1" make an excellent suggestion add a "control" plate without any subject material.

More to keep in mind for future experiments!

Best of luck! And ask questions and let us know how the science fair works out!

Madeline