Some showerheads harbor disease-causing bacteria

What's in your showerhead? Don't wanna know, do you?

Too late, you're reading this — it's disease-causing "mycobacteria" microbes stuck there in their own slime.

"Microbes are everywhere, so in fact finding them in showers is not a surprise," says Laura Baumgartner of the University of Colorado, Boulder, an author of the showerhead survey study. "Finding large numbers of (disease-causing) mycobacteria was a bit of a surprise, though."

Released Monday by the Proceedings of the National Academies of Science journal, the study looked at 45 showerheads in nine cities, including Denver and New York. Using standard genetic tests, the team looked for microbes, expecting to find harmless varieties usually seen in tap water.

They did find those, but surprisingly, discovered about 20% of the samples contained mycobacterium, at least 100 times more than expected, according to the study. That is worrisome because strains of the bacteria cause lung disease, and showerheads "aerosolize" bacteria, making them easy to inhale.

Only about 1 in 4,000 people suffer from lung diseases linked to environmental mycobacteria, says lung expert Theodore Marras of the University of Toronto, but infections have doubled in Ontario in the last decade, perhaps linked to more people showering.

"Given their ubiquity in the water system, the markedly increasing rates of infection, and the extreme difficulty in treating them, I think that this is an important problem," he says in an email.

"For the average person, this isn't a concern, but people with immune (system) disease or cystic fibrosis might want to change their showerheads every six months or so," Baumgartner says. "The first burst of the showerhead is the most concentrated."

The study found one case where cleaning showerheads with bleach actually lead the mycobacteria to proliferate, suggesting they may be resistant to chlorine. "It's anecdotal, but cleaning may paradoxically allow them to grow after other bacteria are killed," Baumgartner says.

Microbes typically grow in "biofilms" on surfaces, allowing them to cling to showerheads despite the onrush of water during a shower. Biofilms grow much better on plastic, rather than metal, showerheads.

Past studies of microbes in showerheads, typically looking for signs of Legionnaires' disease in hospital settings, relied on growing microbes in culture dishes, rather than genetic markers, to identify bugs, Baumgartner says. Many microbes grow poorly in culture, however, which is why the findings surprised the researchers. "We need microbes to survive. Most microbes are good ones," she adds.