The five-second rule: Growing germs for science | Science News for Students

The five-second rule: Growing germs for science

Here’s how to build a low-cost incubator at home
Sep 13, 2017 — 10:17 am EST
Bethany bologna
What's living on this meat and on my kitchen floor? I'm about to find out.

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Many clumsy, hungry people have sworn by the five-second rule. 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). But are bacteria really polite enough to wait five seconds before hopping on board?

We’re putting this five-second rule to the test in the latest DIY Science video. 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.

Before we get to dropping food, though, we need a way to measure how clean or dirty that food becomes. (We also need supplies. Check the end of this post to see a full list of what’s needed and how much it all costs.)

Bacteria are small. We can’t see them with the unaided eye. So how will we keep count? We’ll need to culture any microbes on the food. That means growing them into colonies that are large enough to see.

To do that, we’ll transfer any bacteria from the food onto a substance they would like to eat. We used agar — a gel material made from algae, yeast or animal proteins. It comes as a liquid or a powder. The powdered form must be mixed with distilled water to create the gel. Here’s how:

  • 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.
  • Stir the mix until the agar has dissolved completely.
  • Microwave the mix on high until it comes to a frothy boil (about 45 seconds). Be careful! The glass will be very hot.
  • Take out the glass, stir the contents and then microwave it again until the mixture boils (another 30 seconds). By this point, the agar should be a golden color and smell a bit like meat.
  • Let the mixture cool until the glass is safe to touch.
  • Pour the liquid into petri dishes — shallow plastic dishes used to grow bacteria. The agar should cover the bottom of each dish.
  • Put each dish on a towel to dry, partially covered by its lid. The agar will start to firm in about 10 to 20 minutes.

Once the dishes are dry, they can be used right away or stored in plastic bags in the refrigerator. 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. 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.

Keep it clean!

Bacteria are everywhere. They’re on the floor, in the air and on your hands. 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. 

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). Any glass or spoons were boiled in a pot of water with a little bit of bleach, to ensure they were completely clean. 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.

Lit candles placed around the experiment also helped to keep other microbes away. Candle flames bring in cooler air from below. As it warms, this air rises, creating a small updraft — an air current moving toward the ceiling. This should help prevent germs in the air from settling on the meat or agar.

Make sure you have an adult around if you’re going to work around open flames. Also, don’t play with the spray bottle! Ethanol will cause plenty of misery if it gets in your eyes.

Bologna bombs away!

In our previous post, we determined that we will need six groups of plates — one group for each test condition. We are also making six replicates of each test. That gives us a need for 36 plates. There’s a control with no bologna and a control slice of undropped meat. There is also bologna dropped on clean and dirty sections of floor for either five or 50 seconds.

For the clean section, I wiped down a floor tile as carefully as possible with an ethanol-water mixture. For the dirty floor, I smeared coffee grounds, eggs, vegetable parts and fruit cores onto a tile (definitely the best part). Then I wiped the mess off so the floor tile looked clean.

Bethany swabbing
I swabbed the dropped bologna carefully and thoroughly before swabbing a petri dish.

I cut the lunch meat into quarters and dropped these pieces onto the clean and dirty floor tiles, waiting five or 50 seconds before picking them up. For the clean tile, I made sure to re-clean the tile in between each drop. Each time I picked up a piece of dropped bologna, I rubbed a cotton swab six times all over the side that had touched the floor. For my control — where nothing happened at all — I dipped a cotton swab in a small beaker of distilled water.

I now carefully dragged the cotton swab from each sample across an agar plate in a zigzag pattern. I then turned the plate 90 degrees (about a quarter of a turn) and repeated the zigzag swab. I repeated this turn-and-zigzag action twice more. That ensured complete coverage of the plate. (A diagram for this zigzag swabbing technique can be found here.)

Microbes can be found in almost any environment. But we’re most concerned with ones that might make us sick. These germs will be found among the microbes that can grow at a human body temperature, 37° Celsius (98.6° Fahrenheit). So we need a way to keep our petri dishes at that temperature to let the microbes grow.

That means we need an incubator — a device that keeps a constant temperature. Lab incubators can be very expensive. Cheap incubators meant for hatching chicken eggs are available for about $20. But you build one on your own for even less. This slideshow will tell you how. (Hint: Make the incubator at least a week before you need it because you may need a few days to figure out how many holes it will need to maintain a stable temperature inside.)

DIY incubator slide1

DIY incubator 2

DIY incubator 3

DIY incubator 4

DIY incubator 5

DIY incubator 6

DIY incubator 7

DIY incubator 8

After the experiment, I placed the petri dishes in the incubator, upside down. As the plates warm in the incubator, any liquid in them will start to evaporate. The agar could dry out, and then the microbes might not grow. With the plates upside down, any water will rise onto the agar. Place a cup of distilled water in the incubator. It will keep the air inside humid and microbe-friendly.

Every 24 hours for the next three days, I removed each dish and took a picture of it with a smartphone. Those images will be necessary for counting the colonies.

In the next blog post, you’ll find out how many colonies of microbes grew on those plates.

Materials list

For the experiment
  • 70 percent ethanol ($2.19)
  • Roll of paper towels ($0.98)
  • Permanent marker (to label petri dishes) ($2.97)
  • Nitrile or latex gloves ($4.24)
  • Cotton-tipped swabs ($1.88)
  • Candles ($9.99)
  • 60 x 15 mm sterile petri dishes (two packs of 20) ($6.35 per pack)
  • Glass beakers ($21.70)
  • Nutrient agar ($49.95)
  • Distilled water ($1.00)
  • Microwave ($35.00)
  • Food for dropping (bologna, one package) ($2.99)
  • A digital or smartphone camera
  • Ruler (metric) ($0.99)
  • Small digital scale ($11.85)
  • A book of matches
For the incubator
  • Styrofoam cooler ($7.47)
  • 25-watt lightbulb and wiring ($6.47)
  • Remote digital thermometer ($14.48)
  • Knife ($3.19)
  • Duct tape ($2.94)
  • 28 cm x 35.5 cm (or 11 x 14 inch) picture frame, glass or plastic front only ($1.99)

Is the five-second rule really true? We’ve designed an experiment to find out.

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Power Words

(for more about Power Words, click here)

agar     A gelatinous material made from certain marine algae used as a material (and food source) in which to grow bacteria.

bacteria     (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

bleach     A dilute form of the liquid, sodium hypochlorite, that is used around the home to lighten and brighten fabrics, to remove stains or to kill germs. Or it can mean to lighten something permanently, such as: Being in constant sunlight bleached most of the rich coloring out of the window drapes.

blog     Short for web log, these Internet posts can take the form of news reports, topical discussions, opinionated rants, diaries or photo galleries.

control     A part of an experiment where there is no change from normal conditions. The control is essential to scientific experiments. It shows that any new effect is likely due only to the part of the test that a researcher has altered. For example, if scientists were testing different types of fertilizer in a garden, they would want one section of it to remain unfertilized, as the control. Its area would show how plants in this garden grow under normal conditions. And that gives scientists something against which they can compare their experimental data.

culture     (v. in microbiology) To grow cells outside the body or their normal environment, usually in a beaker, a laboratory dish or some big vessel. To keep the cells healthy, they must be kept at the proper temperature, given the proper nutrients and provided ample room to grow.

current     A fluid — such as of water or air — that moves in a recognizable direction.

degree     (in geometry) A unit of measurement for angles. Each degree equals one three-hundred-and-sixtieth of the circumference of a circle.

digital     (in computer science and engineering)  An adjective indicating that something has been developed numerically on a computer or on some other electronic device, based on a binary system (where all numbers are displayed using a series of only zeros and ones).

ethanol     A type of alcohol, also known as ethyl alcohol, that serves as the basis of alcoholic drinks, such as beer, wine and distilled spirits. It also is used as a fuel, often mixed with gasoline, for instance.

filament     Something with a thin, thread-like shape. For instance, the fragile metal wire that heats up to emit light inside an incandescent light bulb is known as its filament.

gel     A gooey or viscous material that can flow like a thick liquid.

germ     Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

hypothesis     (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

petri dish     A shallow, circular dish used to grow bacteria or other microorganisms.

plastic     Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

protein     Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

smartphone     A cell (or mobile) phone that can perform a host of functions, including search for information on the internet.

Styrofoam     A trademarked name for a type of rigid foam made from light-weight polystyrene plastic. It is used for everything from home craft projects to decorative ornaments and building insulation.

watt     A measure of the rate of energy use, flux (or flow) or production. It is equivalent to one joule per second. It describes the rate of energy converted from one form to another — or moved — per unit of time. For instance, a kilowatt is 1,000 watts, and household energy use is typically measured and quantified in terms of kilowatt-hours, or the number of kilowatts used per hour.