Soap scum is home to opportunistic pathogens. Credit: © Image Source

If you don't scrub your shower curtain, you're asking for trouble. These plastic sheets are flooded with bacteria that can cause nasty infections says Norman Pace, a microbiologist at the University of Colorado, Boulder.

Pace has long been interested in the microbial communities that live all around us, and one day decided to examine the soapy film that covered his shower curtain. "I scraped a little bit of soap scum, put it under the microscope and went: 'Wooah!'" he says. The sample teemed with bacterial life.

To find out whether the bugs were harmless or not, Pace enlisted an undergraduate student, Ulrike Theissen. She collected scum from five shower curtains: Pace's own, three more from colleagues at Boulder, and one from Berkeley, California.

The bugs' DNA showed that around 80% belonged to one of two groups: the sphingomonads and the methylobacteria. Both contain species that are opportunistic pathogens, able to infect wounds or sicken people whose immune systems are suppressed. These include the elderly, or those taking drugs to combat the rejection of transplanted organs.

Each time you take a shower you are engulfed by an aerosol of bacteria, Pace told the American Association for the Advancement of Science meeting in Seattle. In most cases, that will not be dangerous. But if you have an unprotected cut, or your immune system is suppressed, it could be a different story.

The bacteria probably feed on volatile organic chemicals shed from human bodies, says Pace, rather than on soap. "When you cough, belch or fart, you're putting a lot of organic chemistry in there," he says. Chemicals called plasticizers, which make shower curtains more flexible, could also feed bacteria, he suggests.

When you cough, belch or fart, you're putting a lot of organic chemistry in there Norman Pace , University of Colorado

Pace's findings have caused a change in his personal habits: "I take showers, but since the study, I wash my shower curtain every few weeks."

Shower curtains may not be the biggest hazard we encounter while bathing. Other studies have found that the air just above water level in a typical Jacuzzi, or hot tub, is packed with bacteria that can cause lung infections.

"I would not get into a public hot tub. I would not get into a private hot tub, frankly," says Pace.