Science Buddies: "Ask an Expert"

Hello, I recently did an experiment on which common household item is the most germ infested for my science project. I've tried the experiment twice using powder agar and petri dishes. I wiped various items with a cotton swab, following the instructions on the powder agar. The first time, nothing was noticeable except condensation on the lid. I thought I did some of the measurements wrong, so I tried it again. This time, the water seemed to have evaporated and the agar looked like it has cracked. Still, no bacteria was noticeable. I've put it in a warm area inside a box with a towel for 2-3 days. I was wondering if there is something I can do to get this right. Thank you. (And I have until the 20th, so if it's mandatory, I can buy a new set of agar and dishes)

Hi Jenn,

Can you tell me exactly how you made the agar and poured the plates? Did you buy plain agar or nutrient agar? Plain agar won't work because it contains no 'food' for the bacteria. You have to buy NUTRIENT agar because that has sugars, salts, nitrogen source, etc that bacteria need to grow.

Give me the details of what you did to make the plates and i'll try to help you get going on testing things for contamination.

Sybee

Hi. Firstly, thank you for replying. Secondly, I checked and I did use nutrient agar. I followed the instructions provided. Boiled water, added the powder, and poured the mixture into the plates. I let them cool to room temperature then left them in the fridge for about 10 minutes so I could make separate batches. After leaving the plates out at room temperature, I used cotton swabs to get the bacteria and swiped them onto the plates. Appreciate the help!

Hi Jenn,

I wish there was something else I could suggest for you to try, but you seem to be doing everything right. I know that bacteria and yeasts and molds WILL grow on nutrient agar eventually, so keep incubating the plates. There is one thing you can do as a safety precaution if you aren't already doing it and that is to tape the lid to the base of the dish so it can't accidentally come off. Some of the bugs that grow on nutrient agar could make you very sick so it is best not to take any chances with them escaping.

One thing i just thought of--are you moistening the swabs with sterile distilled water before you swab a surface? That might improve your chances of picking up all the bugs that are on a particular surface. Another thing you could do is to sample your room air by leaving the lid off the dish for an hour, then taping it back on.

Keep trying and do let us know what happens. There are LOTS of bacteria and molds out there, so you will see them eventually!

Sybee

Thank you very much for the feedback! I haven't moistened the swabs, so I will definitely try that.

You're welcome! Also be sure to try the test where you leave the lid off the plate for an hour. You could do one indoors and one outside for comparison. Just be sure some squirrel or bird doesn't try to eat your agar!

Good luck!

Sybee

Hello, again! It's been a few days since I tried the experiment again. Sadly, no bacteria.. So, I decided to buy pre-poured agar plates from another company.
I just wanted to ask if bacteria will still grow if the plates have been placed in a place at 110 degrees Fahrenheit for about four hours. (It was left on the radiator) Thanks.

At 110F you would have killed most of the bacteria and the molds except for the spores. Try to keep the plates at around 90F [32C]. The agar is still ok, however, so you could try swabbing the plates again, just don't get them too hot. I know you will succeed--just be persistent (and don't forget to moisten the swabs with sterile water!).

Sybee

Hi Jenn,
Good project!
Are you inverting your plates? Moisture condensing on top of lids may drip down on the surface of the agar and dilute bacteria. As suggested by Sybee, it's important to do controls. Yogurt has bacterial cultures, I wonder if you simply put a drop of yogurt on your plates, would you be able to grow yogurt bacteria- (that would be a positive control)?
Good luck!