We are also making six replicates of each test. In our previous post, we determined that we will need six groups of plates - one group for each test condition. Also, don’t play with the spray bottle! Ethanol will cause plenty of misery if it gets in your eyes. Make sure you have an adult around if you’re going to work around open flames. This should help prevent germs in the air from settling on the meat or agar. As it warms, this air rises, creating a small updraft - an air current moving toward the ceiling. Candle flames bring in cooler air from below. Lit candles placed around the experiment also helped to keep other microbes away. And I used a spray bottle containing 70 percent ethanol - a type of alcohol - and 30 percent water to clean any surface used, wiping everything dry with fresh paper towels. Any glass or spoons were boiled in a pot of water with a little bit of bleach, to ensure they were completely clean. To lessen the chance the experiment would be contaminated, I wore a lab coat and lab gloves (you can buy gloves made of latex or nitrile that you throw away after one use). For our experiment, though, we had to make sure that the bacteria that grew on the plates came only from the dropped food - not from anywhere else. ![]() They’re on the floor, in the air and on your hands. I used a system for mine that included the floor I was testing (clean or dirty), the time (five or 50 seconds) and the plate number. Before you start your experiment, label your petri dishes with a permanent marker to make sure you can keep track of which plate is which. Once the dishes are dry, they can be used right away or stored in plastic bags in the refrigerator. The agar will start to firm in about 10 to 20 minutes. Put each dish on a towel to dry, partially covered by its lid.The agar should cover the bottom of each dish. Pour the liquid into petri dishes - shallow plastic dishes used to grow bacteria.Let the mixture cool until the glass is safe to touch.By this point, the agar should be a golden color and smell a bit like meat. Take out the glass, stir the contents and then microwave it again until the mixture boils (another 30 seconds).Microwave the mix on high until it comes to a frothy boil (about 45 seconds).Stir the mix until the agar has dissolved completely.Place 6 grams (0.2 ounce) of agar powder in a clean glass or beaker and add 100 milliliters (3.4 ounces) of distilled water. ![]() ![]() The powdered form must be mixed with distilled water to create the gel. We used agar - a gel material made from algae, yeast or animal proteins. To do that, we’ll transfer any bacteria from the food onto a substance they would like to eat. That means growing them into colonies that are large enough to see. So how will we keep count? We’ll need to culture any microbes on the food. Check the end of this post to see a full list of what’s needed and how much it all costs.)īacteria are small. ![]() And in our first blog post, we came up with a hypothesis and figured out how many conditions we would need to test in that experiment.īefore we get to dropping food, though, we need a way to measure how clean or dirty that food becomes. We’re putting this five-second rule to the test in the latest DIY Science video. But are bacteria really polite enough to wait five seconds before hopping on board? This is the idea that if you drop a piece of food and pick it up before five seconds have passed, it’s still clean enough to safely eat (at least, if it doesn’t have any hairs or obvious dirt on it). Many clumsy, hungry people have sworn by the five-second rule. You can repeat the steps here and compare your results - or use this as inspiration to design your own experiment. This article is one of a series of Experiments meant to teach students about how science is done, from generating a hypothesis to designing an experiment to analyzing the results with statistics.
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