Science Buddies: "Ask an Expert"

Hello, I'm doing my 6th grade science fair project on the growth of bacteria on grocery bags and how different environmental conditions affect it. I took samples from three types of grocery bags (paper, plastic and reusable fabric ones) and swabbed them with three different food contaminants (poultry juice, egg whites and yogurt whey) and placed my samples in two conditions (car trunk and inside my house). It's been 8 days and very few of the samples have any visible growth of bacteria or mold on them, so I'm starting to worry I won't have any results to discuss by the science fair which is in one week! Do you have any ideas about how to rerun the experiment, maybe using more moisture or more heat? I was expecing to get visible growth on at least some of the samples, especially the ones in the car trunk.

Thanks so much for your help,

Sebastian

Hi Sebastian,

Are the samples you are monitoring the bag fragments themselves? You might not be able to see bacterial/microbial growth with the naked eye. Have you considering swabbing the samples and trying to culture microbial colonies on Agar plates? The danger of reusing grocery bags is not that growth on the bags will get out of control but that they will harbor enough viable bacteria to contaminate food placed in them. You have a perfect opportunity to test this now on the samples that sat for 8 days. Some bag types might harbor more bacteria than others and some locations might be worse than others, which I imagine was your original hypothesis.

About Agar: https://www.sciencebuddies.org/science- ... Agar.shtml
Microbe safety: https://www.sciencebuddies.org/science- ... fety.shtml
More safety: https://www.sciencebuddies.org/science- ... ents.shtml
Interpreting plates: https://www.sciencebuddies.org/science- ... ates.shtml

Cheers,
Colin

Hi Sebastian,
Like what Colin said, it would be better to use Agar plates for bacterial culture. I just want to add one more link for your reference in which the project is similiar to what you are trying in to do.
Germ Invasion
<https://www.sciencebuddies.org/science- ... #procedure >

Hi guys, thanks so much for your help. We got the agar etc today. I have a few questions:

1) Should I just swipe the bag samples or take tiny pieces of the contaminated bags and stick them on the agar?
2) I also have a control group of bag samples that I contaminated withe food and then placed in a bleach or detergent solution. I'm planning on using them on a few Petri dishes too, hoping nothing shows up on them. What do you think?

Thanks again,

Sebastian

I think I would swab the bags not add them to the agar but maybe we'll get another opinion. I like the idea of the decontaminated controls! Good luck!

Cheers,
Colin

Hi Sebastian,

I really like your experiment and just want to throw my two cents in. Get a clean bag from the grocery store and use that as a control along with your decontaminated bag. I am curious to know how clean the bags are that have not been used.

You will probably get some molds growing on the agar that you swab as well as bacteria because mold spores are everywhere and don't die as readily as most bacteria. You can tell a mold by its fuzzy appearance on the agar. Bacterial colonies are usually shiny and fairly small. Just remember to tape the lids of the plates to the bottoms so that they can't come open accidentally, and DO NOT open them after the bacteria and molds have grown up (3-4 days). Some of these guys could make you VERY sick, so don't open the plates. Dispose of them by putting the whole plate into a pail of 10% clorox and leaving them overnight. Then put them in the trash. The clorox will kill all the microorganisms.

Let us know how it turns out, OK?

Sybee

Achiara wrote:Hello, I'm doing my 6th grade science fair project on the growth of bacteria on grocery bags and how different environmental conditions affect it. I took samples from three types of grocery bags (paper, plastic and reusable fabric ones) and swabbed them with three different food contaminants (poultry juice, egg whites and yogurt whey) and placed my samples in two conditions (car trunk and inside my house). It's been 8 days and very few of the samples have any visible growth of bacteria or mold on them, so I'm starting to worry I won't have any results to discuss by the science fair which is in one week! Do you have any ideas about how to rerun the experiment, maybe using more moisture or more heat? I was expecting to get visible growth on at least some of the samples, especially the ones in the car trunk.

Thanks so much for your help,

Sebastian

Hi everyone,
Thanks for all the help on my experiment. In the end, I got no results whatsoever. However, I performed an experimental survey and was able to use that data to supplement my bacteria data. I was wondering if there was anything I could do next time, because I got second place and qualified for the county science fair.

Sebastian

Hi Sebastian,

Do you mean that no colonies grew on your agar? Sometime this can happen if the incubation conditions are not ideal. How did you incubate the plates? Also, I'd like to say that it is not uncommon for experiments in science to not yield meaningful results. It happens to people with PhDs all the time! The important thing is to learn something so you can better design the experiment in the future. Let us know exactly what you did and what you think might explain your results and we can try to help you understand them better.

Cheers,
Colin

I created Petri dishes with agar designed for bacterial growth, following the directions given. Then I used damp Q-tips to swab each different grocery bag/contaminant sample. We labelled and sealed the dishes with tape then set them on top of a fridge for 5 days until the fair. We actually kept them for 5 days longer just to see if anything would become visible, and we got small amounts of mold on some. I didn't incubate the dishes upside down, could that explain things? I also think that when we originally set up the samples with yogurt, poultry juice and egg whites, we may not have used enough on the samples, since we just swabbed the liquid on there and maybe most of it evaporated? I'm hoping to re-run the experiment so I'd love your feedback and help! Thanks!

Hi,

This all sounds pretty good. My guess is very little lived on the bags you initially tried to contaminate. You may have done nothing wrong! Your results could indicate that there were very few viable bacteria on the bag samples. Next time, as a control, you should swab samples directly from the eggs, poultry juice, and yogurt whey as well. It is also possible that the foods were cleaner than expected. Salmonella is actually very rarely found in eggs (less than 1 in 10,000 I think) though I would expect the yogurt sample should have grown something if the label said it had active cultures in it. Saliva is another good control for viable bacteria to make sure nothing is wrong with your culture conditions.

Cheers,
Colin