Staphylococus epidermidis is a common form of bacteria found on human skin. Invisible to the eye, bacteria are revealed when a handprint made in agar gel (above) is cultured in a laboratory.
The surface of human skin is crowded with bacteria that would, if your immune system allowed it, cause serious infection. But researchers at the University of California, San Diego, have discovered that one bacterium in that mix, Staphylococcus epidermidis (a close cousin of methicillin-resistant S. aureus, or MRSA) may actually help fine-tune the immune system. To do its good work, S. epidermidis deploys a molecule that blocks aggressive inflammatory agents. If unchecked, those agents would ignite a rashy reaction around even a minor scrape. Good staph stays good only if locked outside by the skin's multiple defenses, though. Give that germ a way into a weakened body - like a ride on a surgical implant - and it can ignite a potentially fatal infection.
(National Geographic, June 2011)