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An Explorit "Science Bytes" article by Jennifer LeBlanc (1996)

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by Jennifer LeBlanc (1996)

The human body is host to an active ecosystem. We each have microbes such as bacteria, viruses, fungi and protozoa in our mouth, eyes, ears, nose, intestines and skin. Although it is popular to believe microbes are dirty, disease-causing creatures, this is a highly biased and narrow view of microbial life. Microbes perform many important functions for us, producing vitamins, helping digest our food, and working as an essential part of our immune system.

Our skin is a rich source of microbes. Adults have between 11,000 (forearm) and 1.5 million (scalp) bacteria per square centimeter of skin. Babies are born microbe-free, but rapidly pick up their own set of microbial friends and reach an adult level of bacterial inhabitants within nine days.

This bacterial colonization is essential for immunity as these resident microbes defend their territory against disease-causing microbes. In one experiment, the typhoid bacteria, Salmonella typhi, was smeared on human skin and, within twenty minutes, was completely destroyed by resident bacteria. The same Salmonella tyhpi smeared on a glass slide, after twenty minutes, had remained and reproduced. Constant battle and territorial defense occurs on our skin. We owe much of our general good health to the results of these bacterial wars.

There are two main types of microbes on our skin: permanent residents and transients. Many people believe washing will remove these "bugs and germs." In fact, studies show even the most vigorous washing does not remove all our special flora.

Permanent microbial species live on the surface of the skin and also in deeper layers. Washing off the surface populations simply encourages microbes in the lower layers to come and set up camp. However, transient populations, which are picked up from other people or animals, are generally removed by washing.

So what happens to all those microbes when we shed our skin cells? Unlike our beautiful reptiles here at Explorit, we humans are messy shedders. We constantly shed skin cells everywhere. Every day, we produce 10 billion skin flakes, called squames. In a year, these dead, discarded skin flakes weigh in at over two kilograms.

Where do all these squames go? Dust. Dust is 90% skin flakes, which are the favorite food of the peaceful dust mite. They have evolved with humans to live in protected areas our beds and carpets. The average double bed has 2 million dust mites. And yes, your bed has them. Numerous epidemiological studies have found that 100% of homes have dust mites. These peaceful squames-munchers are the final link in the skin's ecosystem.

Fortunately for our mental health, we humans don't see very well. We can't visibly detect the millions of 0.4 millimeter dust mites cleaning up our mess of squames. We don't see or feel our tiny (0.1-0.4 mm) resident skin microbes, wriggling around, defending their territory against other microbes.

But maybe if we did see and understand them, we would accept and appreciate their essential contribution to the only real home we have, our body.



  • Andrews, Michael. 1976. The Life that Lives on Man. New York: Taplinger.
  • Bodanis, David. 1986. The Secret House: 24 hours in the strange and unexpected world in which we spend our nights and days. New York: Simon and Schuster.
  • Lewis, Rick. 1992. The Bugs Within Us. FDA Consumer, v. 26(7): 37-42.
  • Sagan, Dorion. 1988. Garden of Microbial Delights: A Practical guide to the Subvisible World. Boston: Harcourt, Bruce and Javonovich.
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