NEVER HOME ALONE

From Microbes to Millipedes, Camel Crickets, and Honeybees, the Natural History of Where We Live

By Rob Dunn

Illustrated. 323 pp. Basic Books. $30.

A young man I know used to say that humans have no need to bathe, since our hair and bodies are designed to self-cleanse. I would fight him on it, being of the opinion that washing up occasionally was good for us — and for the people with whom we lived. But now, after reading the entomologist Rob Dunn’s description of the myriad microbial life-forms that take up residence in a typical American showerhead, I’m starting to think maybe that young man was onto something.

With an army of collaborators, Dunn, a professor of applied ecology at North Carolina State University, took samples of the gunk inside hundreds of showerheads, and found a profusion of microbial fauna. Tap water itself, he writes in the chatty, informative “Never Home Alone,” teems with amoebas, bacteria, nematodes and crustaceans. As the water passes through the showerhead, these microbes lay down a kind of scaffolding known as biofilm to protect themselves from getting washed away with every ablution. They make the biofilm “out of their own excretions,” Dunn writes bluntly. “In essence, by working together, the bacteria poop a little indestructible condominium in your pipes.”

It gets worse. Filtered through that poop-biofilm, the water that washes over you, as you supposedly scrub yourself clean, might contain not only all those harmless amoebas and nematodes but a few bacteria that can be dangerous — in particular some species of Mycobacterium, cousins of the Mycobacterium that causes tuberculosis. And the pathogens are there because we provided the perfect breeding ground for them, when we tried to purify our tap water in the first place. Municipal water treatment plants use chlorine and other chemicals that kill off the bacterium’s natural predators, allowing Mycobacterium to thrive. Tap water that comes from a well, in contrast, has never gone through a treatment plant and has a rich microbial life. It might look more dangerous, but it’s actually safer, Dunn explains. All those organisms in well water are themselves harmless, and they tend to fight off the potentially dangerous ones like Mycobacterium — that’s how biodiversity works.