Are the germs in your shower harmless?

They did tests on 45 different shower heads from nine cities around the world, using genetic testing to reveal bacteria that can’t be detected by the usual method of growing them in a dish.

One type of bacteria, called mycobacteria, appeared in higher numbers inside shower heads. The researchers think these bacteria form a waxy biofilm that’s not easily washed away by water.

The most noteworthy finding was a species called Mycobacterium avium. About 20 per cent of the swabs gave test results suggesting that Mycobacterium avium could be present. Mycobacterium avium does have the potential to make people ill, although the immune system of a healthy person will almost always keep this germ in check.

Could my shower head make me ill?

For most people, the answer is a straight no. For bacteria in a shower head to pose a danger, first you’d have to either swallow them or breathe them in.

When the researchers tested water that was sprayed out of the shower heads, they found bacteria levels reflected those in the water supply, not those in the shower head. So, it seems that any bacteria that build up in the shower head don’t make their way into the spray from the shower.

Mycobacterium avium isn’t usually a danger to healthy people, although it can cause lung infections in people with a damaged immune system. This includes people with cystic fibrosis or AIDS, and people who’ve recently had an organ transplant.

What should I do now?

The researchers suggest that people with a damaged immune system might be better off having baths rather than showers. If Mycobacterium avium is present, the spray from a shower may make you more likely to breathe it in. However, the researchers didn’t test this idea.

If all this talk of bacteria has you worried, think twice before you rush off to bleach your shower head. The researchers say that mycobacteria will simply grow back. When they did try treating one shower head with diluted bleach, they ended up with a greater proportion of some (harmless) bacteria in subsequent tests. That may be because any bacteria that stick around in a shower head are immune to the chlorine used to treat municipal water, so might also be resistant to diluted bleach, which is also chlorine-based.