DIY Surface Bacteria Sampling Dish | Vegan

If you follow me on Instagram (#mhsfcs, #washurhands) you may be wondering why we are decorating for Christmas so early. See all of the beautiful reds and greens?

Although we would love a break from this hot muggy weather, Christmas is a long time away. Those colors you see are all kinds of microorganisms growing on surfaces in our school.

I read a lot online about testing surface bacteria with agar dishes and tape and knew the experiment would be perfect for my students to demonstrate the importance of hygiene in food preparation. I quickly found it to be cost-prohibitive. After looking through several recipes online I came up with this concoction. Because it is so quick, cheap, and easy to make, we were able to collect samples from 54 surfaces in our school!

To make four dishes, dissolve 1 tsp guar gum and 2 tsp sugar in 1/2 C hot water. Whisk vigorously with a fork because the guar sets quickly. Carefully pour the mixture into four 7" heavy duty plates.* Allow to set and place in quart-sized zipper-seal plastic bags.

*Although it sets up well, the guar gum is still a bit runny. In the future I think I will cut clear Solo cups for trays. This would also allow for the use of a microscope. They would also fit in a snack or sandwich bag which are less expensive than quart-sized.

Like most good ideas, this one came late at night. All of the recipes called for gelatin which I do not use in my home. Guar gum is a great substitute that I have on hand. It is six times stronger and at least that more cheaper than gelatin. A little goes a long way.

To test a surface, students tore 1 square of very cheap 2-ply toilet paper and wiped the whole surface.  They placed the paper "bacteria side down" in the guar gum. Then they very carefully peeled off the top "ply" of toilet paper. They put the dish back in to their quart bag and observed the bacteria multiplying over the next several days.

This is my favorite kitchen safety and sanitation activity to date. It was perfect at proving what I wanted, which is:

• Places you think are gross, like bathroom floors or school lunch tables, are the ones most carefully cleaned.
• The dirtiest places are the ones you don't think about like cell phones, back packs, and shoes.
• Sanitize your countertops before starting a lab, even if they appear clean.
• For gosh sakes, put that phone away!!!

Hopefully this recipe puts a fun petri dish surface bacteria activity in reach for your students! Be sure to comment back here to share!