Science Buddies: "Ask an Expert"

Hello,

I have been working on an experiment where I test the effectiveness of hand soap versus hand sanitizer. I touched an iPhone and a laptop keyboard before washing my hands for 20 seconds, then used a sterile cotton swab to transfer the bacteria into a petri dish by swabbing my hands and then drawing a line across the agar. They have been incubating for a week now and still have not shown any results. I have not been able to find the issue in the process. I put the nutrient agar in a pressure cooker for half an hour before pouring it into the petri dishes. I may have let them sit for too long before swabbing them, or the sterilization process could have been wrong, or maybe I swabbed them incorrectly.

Any ideas?

Hi!
It sounds like you're putting a lot of effort into your experiment! Here are some things to consider:
Swabbing: Ensure the swab is moist and you're swabbing thoroughly. Make sure to streak the swab across the agar in a zig-zag motion to transfer enough bacteria.
Incubation: Check the temperature (around 37C/98.6F is ideal) and make sure the dishes aren’t exposed to contamination.
Sterilization: Ensure the agar isn’t too hot when poured and all tools are sterile.
Bacterial Load: If there was low bacteria to begin with, it might be too minimal to grow. You might also try using a control sample.
Hope this helps!