Your skin is your largest organ, the boundary between you and the world, a key part of what makes you who you are. But, despite what you may think, you’re not the only one who lives in it.

In fact, according to the National Institutes of Health, your skin is a virtual microbial zoo–home to a wide variety of bacterial species. Over the last decade, researchers have begun to study this microbial menagerie through molecular study of the skin and its microscopic inhabitants. No need to reach for the antibacterial soap, though. Most of your bacterial borders are harmless, and some are downright helpful.

The Non-Enemy Within

Surprised? Don’t be. Bacteria turn up everywhere life does, and some places other life doesn’t–from the darkest depths of the ocean to the insides of your intestines. And though the unicellular organisms are best known for causing diseases from tooth decay to tuberculosis (the latter of which kills more than a million people per year), that isn’t quite fair. A few bad bugs actually give countless benign–and even beneficial–bacteria a bad rap.

In fact, researchers say we couldn’t survive without the bacteria that “infest” us. Hard to believe? Consider this: the bacteria inside your body likely outnumber your own cells, no matter how you scrub. Previous estimates said that “our” bacteria outnumber our own cells by a 10:1 ratio. Those estimates have recently come under attack, but researchers still think the bacteria that call you home outnumber your own 30 trillion (or so) cells by several trillion, at least most of the time. On average, your body’s own cells are significantly larger than the  bacteria that call you home, but still.

Even familiar infection-causing bacteria like Staphylococcus aureus (pictured above) normally don’t cause trouble. Microbiologists say around 25 percent of the population currently carries staph, but most of them aren’t sick from it.

Simply put: bacteria can and do cause nasty infections and illnesses, but doing so is hardly their defining property.

Bacterial Basics

So what is their defining property? Think back to Biology 101. One of the most basic divisions in biology is the one between eukaryotic and prokaryotic cells. Basically, a eukaryotic cell includes a nucleus that holds its DNA. A prokaryotic cell has no such nucleus.

Animals, plants, and fungi are built of eukaryotic cells. Most prokaryotes, on the other hand, are unicellular organisms. And bacteria are the signature prokaryotes. In fact, “bacteria” and “prokaryote” were once nearly synonymous, until enterprising biologists carved out a different class of prokaryotes called “archaea.”

Still, bacteria represent one of the major branches on the tree of life. Microbiologists estimate that there are 1 nonillion of them on the planet. Nonillion? That’s a 1 with 30 zeroes after it:

1,000,000,000,000,000,000,000,000,000,000.

Maybe we’re the ones infesting their world.

* Originally published in KnowledgeNews