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Skin Shield

Science Snack
Skin Shield
Explore the body’s first line of defense against pathogens.
Skin Shield
Explore the body’s first line of defense against pathogens.

How does your skin keep you safe? In this Snack, a tomato lets you see what happens when you breach this line of defense.

COVID-19 Learning Note: Your skin and mucous membranes are the first line of defense against viruses, including the one that causes COVID-19. To become infected, the coronavirus has to come into contact with your respiratory cells, which is why it’s important to wash your hands, which could be harboring viruses, before touching your face.

Video Demonstration
Skin Shield - Science Snack Activity
Tools and Materials
  • Two similar-sized tomatoes (nectarines, plums, or similar soft fruits will also work)
  • A plate or tray (juices may run!)
  • Toothpick
  • Hand lens 
  • Optional: Digital scale (not shown) and ruler
  1. Put the tomatoes on the plate or tray.
  2. Poke six holes in one of the tomatoes with a toothpick. Leave the other tomato alone.
  3. Put the tomatoes in a safe place at room temperature where they won’t be disturbed.
To Do and Notice

Note: Be sure to wash your hands after handling the tomatoes, and do not try tasting them!

Watch the tomatoes every day for at least a week. Record your observations. How do the poked and unpoked tomatoes differ? How does color, smell, texture, and shape change? 

Take measurements with a ruler and weigh the tomatoes with your digital scale (if you have one). 

Use a hand lens to look carefully at the surfaces of both tomatoes. What do you see? What organisms can you find living on each? How do the size and color of the “infections” in the poked tomato change over time? 

You’ll probably see significant differences between the injured and uninjured tomato. Notice that the injured tomato has likely shrunk and lost mass, and the sites of injury have probably become colonized with a variety of fungi and other microorganisms. 

When you’ve concluded your experiment, compost the tomatoes or throw them away. 

What’s Going On?

This experiment shows the important role of the skin as an organism’s first line of defense against pathogens. Like an “injured” tomato, a cut in your skin can become infected by pathogens, too. Bacteria—and sometimes fungi—can enter through the cut and take up residence in your tissues, causing an infection. Your intact skin is the primary structure protecting you from these kinds of infections.

The outer layer of your skin, or epidermis, forms a physical and chemical barrier that most pathogens can’t penetrate. (The mucous membranes lining your airways and gut have a similar role to play inside your body.) The cells of the skin are joined tightly together, and they’re full of a protein that makes them tough and resistant to bacteria. Your skin is also salty and slightly acidic, making it a less hospitable environment for microorganisms. 

The skin of the fruit plays an identical role for the plant. Plant skin, or cuticle, contains wax and other molecules that help it retain water and prevent invasion by pathogens. When you injured the fruit by poking holes in it, microbes in the environment jumped at the opportunity to invade and colonize within the exposed tissue. You may have observed molds, bacteria, and various protists and nematodes growing in your injured fruit. 

Even after injury and infection, plants still have ways to protect themselves. Plant cells produce a variety of antimicrobial enzymes and other molecules that can help inhibit the reproduction of microscopic pathogens, although they don’t target specific pathogens. The human body also has non-pathogen-specific defenses that respond to cell damage caused by infection. These systems are all part of the innate immune system. All plants and animals have versions of these systems.

Going Further

Try this experiment using different types of fruits and see whether the course of their infections differ. 

Design an experiment to answer a question: Does the size of the wound matter? If you treat the wound with a bandage or antiseptic—as you would with a cut on your finger—does this change the course of the infection? How do you think these treatments work? 

Measuring and weighing the fruits over the course of the experiment can give you more information about the effects of injury on plants. If your fruit changes size and mass in response to injury, what do you think is going on? 


This Snack was adapted from an original activity from Katherine Ward and T. Holtzclaw.