Microbe hunting

Cucumbers.  Image by Image by congerdesign from Pixabay

Bacteria, yeast, and other microbes are a necessary part of natural part of the world.  Microbes are microscopic, meaning that we cannot see them with the naked eye.  Instead, we hunt for microorganisms in our homes using petri dishes filled with a solid nutrient agar.  When the microbes are plated on a solid surface, each cell gives rise to a visible mass of cells called a colony which starts from a single cell.  Different species of microorganisms will produce unique colonies – the shape, color, texture and size of colonies can vary between microbial species.  The appearance and characteristics of a microbial colony is known as its morphology.  These distinguishing features are an important tool for scientists to differentiate between species of microorganisms.

Microbes can be harmful or helpful.  Beneficial bacteria and yeast are necessary to ferment vegetables, make bread rise, and turn milk into cheese.  The collection of microbiota that live in and on our bodies can contribute to our health by producing important vitamins and helping our digestive systems to metabolize food.  These same microorganisms fight off disease-causing bacteria, or pathogens.  The spread of pathogens can be limited using household cleaners like vinegar, bleach, and quaternary ammonium compounds (known as ‘quats’).  If a person has a bacterial infection, they will be prescribed antibiotics to combat the infection.

Using Edvotek Kit 161, we can explore the microorganisms in our environment and the effects of household chemicals on those microorganisms.  The kit includes enough reagents to make 20 agar plates, so there is a lot of exploring to be done!  (New to pouring nutrient agar plates?  We have a video tutorial that will help you!)

To prepare your microbe hunt:

Microbes cultured from the air.  CDC/ Dr. Libero Ajello

It’s easy to find microbes in your home!  There are a few ways that we can transfer the microorganisms onto our nutrient agar.


  1. Air analysis – Remove the lid from a fresh nutrient agar plate. Place the plate, agar side up, in a location in your home or outdoors.  Allow the plate to sit out for at least eight hours (overnight works well).  Replace the lid.
  2. Water analysis – Cool nutrient agar to 60°  Add 1 ml of water sample to empty petri dish.  Swirl to mix the media and water.  Replace the lid and allow the agar to solidify.
  3. Surface analysis – Rub a surface (sink handle, toilet seat, etc.) with the cotton end of a sterile swab. Using a gentle rolling motion, rub the surface of a fresh agar plate with the swab.  Replace the lid.

To encourage the microorganisms to grow, incubate the plates at room temperature for 2-3 days, or until you see colonies forming.  (If you have access to a 37°C incubator, the microbial colonies should grow in 24 hours).  Don’t forget to prepare a control plate!  A scientific control is a part of the experiment that eliminates the variable (in our case, introduction of the bacteria).  If our control plate grows bacteria, that means that our nutrient agar plates were contaminated, which would influence the results of our experiment.

After incubation, examine your plates.  Individual colonies range from 1 mm to 5 mm in size. The edges of individual colonies can appear wavy, serrated, or smooth. The textures of the colony may appear mucoid (slimy or gummy), smooth (shiny, uniform texture) or rough (granulated texture, matte surface). Some microbes produce pigments, which results in colonies in a wide range of colors (including pink, yellow, and white). The colony morphology allows scientists to make preliminary species identifications that are confirmed using the Polymerase Chain Reaction (PCR) or other diagnostic techniques.Screen Shot 2020-06-03 at 4.32.14 PM

To test household cleaners:

161_media-1The Kirby-Bauer test (KB test) tests the susceptibility of microbes to different antibiotics and cleaners.  Using the Kirby-Bauer method, we can determine which household cleaners are effective against bacteria.  Bacteria are plated onto the nutrient agar plate.  A paper disk soaked in antibiotic or cleaner is placed onto the plate, and the experiment is incubated for at least 24 hours.  If the antibiotic or cleaner is effective, a clear ring will surround the paper disk.  Effective microbicides will have a larger clear ring than those that has smaller regions of clearing.

In addition to household cleaners, many food products are believed to be natural antimicrobials.  Using this set-up, you can test natural products like onions, garlic, or honey.

A further extension would be to add some of the disks to the microbes that you cultured from the environment!

After performing these experiments, we recommend sanitizing your plates with bleach before disposal.  Immerse petri plates, open tubes and other contaminated materials into a tub containing a 10% bleach solution. Soak the materials overnight and then discard. Wear gloves and goggles when working with bleach.

Enjoy your microbe hunt!

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