It's warm and damp and dark -- the perfect place for bacteria to nestle and stay for a while.
It turns out that that's just what they do -- in your showerhead.
What's more, says a new study, the mucky film of microorganisms lining the inside of your showerhead often harbors bacteria that can cause lung disease, including a cough, fever, fatigue and weight loss.
These mycobacteria -- close relatives of the bacterium that causes tuberculosis -- can be more than 100 times more prevalent in showerheads than in the water in the pipes just upstream, according to the research, published in Proceedings of the National Academy of Sciences.
Infections with such non-tuberculosis mycobacteria have risen in recent years, up six fold since 1997, according to another study. The bacteria don't threaten healthy people, but those with cystic fibrosis, AIDS, recent organ transplants or other immune-compromising conditions are at risk of an infection.
"There's been a growing voice in the medical field hypothesizing that showering has caused some of this increase," said the study's lead author, Leah Feazel of the University of Colorado, Boulder. "One hundred years ago, people bathed, they didn't shower."
The problem is not just that the microorganisms are enriched in the showerhead, Feazel said. It's also that the spray nozzle creates a fine mist of tiny water droplets. "These tiny, tiny particles can go all the way into your deep lungs," she said.
"Most of us are in the shower long enough to inhale a fairly reasonable amount of mycobacteria," Joe Falkinham of Virginia Tech in Blacksburg told Discovery News.
Feazel and colleagues isolated and sequenced DNA from showerheads and compared the findings with a database of DNA sequences to determine what organisms were living in the showerhead.
The study considered 45 different sites in nine U.S. cities, sampling several sites repeatedly to get a feeling for how microbial communities changed over time.
"The results showed that there's a very complex community inside most shower heads," Feazel said. "There are lots of different species. One of those is Mycobacterium avium, which causes pulmonary disease in people who are immune compromised."
Mycobacteria have a waxy exterior that makes them prone to stick to surfaces and join mats of microorganisms known as biofilms. Even the small amount of organic matter remaining in water supplies is enough to feed these organisms.
If you are scrubbing out your showerhead already, put the sponge down.
Disinfecting the showerhead might make things worse, the researchers found. They tried cleaning one showerhead with bleach and found that it carried even more mycobacteria after the cleaning than before. "Mycobacteria are known to be very chlorine resistant," Feazel said. "By using bleach, we probably killed everything else."
The researchers recommend that those people who are at risk of infection take baths instead of showering. Metal showerheads appear to foster less growth than plastic showerheads, although many showerheads that look metallic are actually plastic. Another option is to buy a cheap showerhead and change it every few months Feazel said.
It's also a good idea to open the window while you shower, Falkinham said, and to keep your water heater above 130 degrees, which he acknowledges runs counter to energy-saving advice to turn the heat down.
"Once you're infected with mycobacteria, you're always infected," he said. "The drug therapy requires multiple drugs. If a patient can tolerate the drugs, they'll get rid of the disease symptoms. They'll never get rid of the organisms."
Unlike tuberculosis, these mycobacteria are not transmitted from person to person.
While it might make us feel a little dirty to know what is lurking at the very source of our washing up, the presence of these microbes is better than skipping the chlorine treatment in our water systems, Falkinham notes.
"These are normal inhabitants that, as we have cleaned up our environment, now find a niche in which to grow," he said, "but it's a lot better than having a water system that has shigella or salmonella or one of the real pathogens." <