Earlier this week, the news media picked up on a story in the Proceedings of the National Academy of Sciences. Basically, the story showed that Mycobacterium avium and related Mycobacteria (but, happily none associated with TB) were present in the showerheads of homes across the US. In and of itself, this shouldn't be a surprise, M. avium is present in the water supply, so there's no reason for it not to be in showers. The more interesting result was the level, relatively concentrated, and well above the background. Not only that, but when the researchers tried bleaching the shower heads, this only made the problem worse. M. avium is resistant to bleach, so there's a selection that goes on, killing other organisms and leaving a population of cells that's largely made up of this and related species.
So far, not a lot to concern anyone, unless they're licking their showerheads. The problem comes with aerosols. Aerosols are small droplets of water (or any material really) that float in the air. Thse can contain bacteria, and can be inhaled. Suck these droplets into your lungs (the idea goes) and maybe you'll take in some of the dangerous bacteria at the same time. If you're immune compromised (through being very young, very old, or having a damaged immune system) and there's a chance you'll develop lung disease.
That was the story in most of the media, and it generally ended with the advice to change your showerhead every 6 months or so, and to replace what you have now with a metal showerhead if yours is made of plastic.
But that's not the full story. In the paper itself, the researchers did look at aerosols, and found that the levels of bacteria were no higher than in the water that went into the shower in the first place. Their suggestion is that the bacteria are cleared in the first few moments after the shower is turned on, but as the paper suggests, this needs a lot more research before it can be confirmed. It's also possible, although not discussed in the paper, that bacteria in the showerhead aren't released as aerosols at all, but again, there's no real data to confirm or reject this.
The other technical consideration is that the detection in this study was indirect. Not that this is a problem with the study itself, detecting microbes from the water column is incredibly difficult. But it is a proxy measure (in this case looking at RNA), which doesn't tell you everything about the organism. For example, these bacteria are living as part of a biofilm (a complex community of bacteria living on a surface), and we know that biofilm living is very different than free living, which may affect how ready these organisms are to cause infection. Again, it doesn't prove that there's nothing going on, but does indicate we need more research to determine what the presence of these organisms really mean.
Overall, the paper is a good one, and it does remind us that we live in a world that's full of microbes. That's important to remember, and certainly for the immunocompromised it has to be taken very seriously, and we should respect the bugs around us. But at the same time, there's no real reason to live in fear. What our parents taught us as kids: wash your hands, sneeze into your sleeve, and don't spit on people, are the kinds of rules that will keep us clean. We've evolved for millions of years to live with bacteria, and we wouldn't survive without them. In fact, over cleaning can cause more problems than just carrying on with life.
So keep showering! If nothing else, it will prevent you from offending your friends.