Expert: John Locke - 3/6/2010

Question
Is it true that 49 bacteria live on every square inch of a toilet seat, 411 live on every square inch of a trash can, and 32 million live on every square inch of human skin?  I read it.

Answer
Thanks for using AllExperts. I can't tell you whether these numbers are correct offhand without a source, but they don't seem unreasonable, and they hint at a fact that you must find disconcerting: more bacteria live on the surface of human skin than some objects that we consider filthy, like toilet seats and trash cans (see this Google search for many pages discussing the topic:
http://www.google.com/search?hl=en&safe=off&q=toilet+seat+bacterial+count&aq=f&a...).
What gives?

Well, for one, the mere presence of a large number of bacteria does not indicate that something is dirty or dangerous. Bacteria are able to adhere to and thrive on certain surfaces much better than others--human skin, for example, makes a much better surface than plastic. Hence, you will always find more bacteria on certain surfaces, just as you would find more animals in a hospitable environment compared to a non-hospitable one. Also, surfaces that come into contact with other surfaces often are more likely to have high bacterial counts. Large numbers of bacteria are found on the hands simply because they touch things all the time.

Second, we have evolved significant defenses against these colonizing bacteria to the point where they pose us essentially no threat so long as our skin is intact and our immune systems competent. Third, the bacteria found on human skin are unlikely to cause disease under normal circumstances (again, assuming that one's skin is intact and immune system functioning), but the populations of bacteria on other surfaces may be more likely to cause disease. The bacteria found on a toilet seat, for instance, heavily represent those present in the feces, and someone who has a gastrointestinal infection could easily contaminate a toilet seat with bacteria that could harm someone else. Such bacteria would not be present on the person's skin, however.

So the fact that one surface is dirtier than another doesn't tell use the whole picture. It does suggest that the basic safeguards against getting sick still apply: regular handwashing and good hygiene.