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Illustrating the threat of emerging diseases

October 18, 2000
Web posted at: 12:31 PM EDT (1631 GMT)

Perhaps the single most important point in teaching about emerging diseases is to impress on students the ubiquitous nature of microorganisms. Bacteria, viruses and fungi are literally all around us.

While growing viruses is relatively complicated and expensive, growing bacteria is not. We can use this as a way to show students there are microorganisms everywhere. It can then be relatively easy to connect the ideas that bacteria or viruses may be released from a hiding place when people enter areas never before inhabited by humans.

If a virus or bacterium that is usually found in some other species in nature can successfully infect a human, it is then released into the human population. Our objective is to help students understand where bacteria are to be found (everywhere) and how easily they can be passed from one living thing to another.

Experiment

To grow bacteria, you need to obtain sterile nutrient agar plates made for growing bacteria and some sterile cotton swabs. Have students sample areas around the classroom by rubbing the cotton swabs on a selected surface and then on a marked area of an agar plate. One plate can be used for several samples since we are not interested in isolating any single microbial strain.

It’s important to impress on the students that touching the cotton end of the swab will contaminate the sample (unless it is their hands they are sampling) and render the experiment useless. Although most students will think of it automatically, make sure they sample doorknobs, especially those for restrooms. Make sure you leave a plate, or area of a plate, as a control to ensure that there are no bacteria already present on the agar. You might even consider sending students outside for sampling to determine whether there are differences in bacteria sampled from different locations.

To show transmission of organisms by air, have someone sneeze on a plate and another cough on a plate. If you have enough plates, leave one with the lid off in the middle of the room for 10 to 20 minutes.

To show that organisms are transferred by hand, have a couple of students thoroughly wash their hands, then touch the surface of the agar plate. With the same part of the hand that touched the agar, have them touch the hand of someone who has not washed his or her hands for a while, then touch a different spot on the plate. Have the unwashed person touch yet another spot on the plate. You might even examine the differences between wash preparations, i.e., using a common bar soap vs. a surgical scrub such as 4 percent chlorhexidine.

After inoculating all the plates, incubate them at 37 degrees Celsius overnight. Access to a commercial incubator would be preferred. If one is not available, experiment using an insulated box containing a 25-watt light of some kind or some other source of heat, such as a heating pad. If these options are unavailable, simply leave the plates in as warm a place as you can find but not in direct sunlight. The closer you can get to 37 C, the better the bacteria will grow.

Results

Since bacteria are so small individually that they can only be seen with a microscope, we use plates to grow "colonies" of bacteria that are large enough to see. Each colony contains millions of bacteria that originated with a single bacterium deposited on the agar surface.

The colonies will differ in size, color and even shape to some extent because of the different varieties of bacteria present. What you see will depend on where the samples were obtained and how many different bacteria were present in the sample. Most of these strains are harmless but ever-present.

You should also see an interesting phenomenon with the washed-hands experiment, which will show how nearly impossible it is to remove all the bacteria by washing. It is also shows how easily bacteria are spread from person to person.

Again, emphasize the concept that microorganisms, including viruses, are everywhere! Students should be able to see this from these experiments. No matter how clean a surface may appear to be, bacteria and viruses are still present. That is why surgeons work so hard to create a sterile environment for surgery.

Because these organisms are everywhere, our bodies are under a constant threat of attack. We need to be careful to avoid giving these microbes a chance to cause disease. We need to be aware that there are bacteria and viruses present, even in a previously undiscovered area of the world where man has never been before. If we are not careful, we could provide a way for those new organisms to enter the world we inhabit.

Reference

Perry, J.J., and Staley, J. "Microbiology : Dynamics and Diversity," Second edition. Saunders College Publishing, 1997.