Rhodotorula mucilaginosa

by Rachel Adams

Sometimes the invisible things in your home become visible, and you rather wish they wouldn’t.

Take that pink stuff growing in your bathroom. On your shower curtain, on the bathtub grout, or even on your toothbrush, there might be a pink film slowly expanding and adding a little decorative color to your life where it is not otherwise invited.

That growth is most likely Rhodotorula mucilaginosa, a type of fungus known as a yeast that can grow as single cells and reproduce by budding. (I say most likely, because there is another pink film in bathrooms, a bacterium called Serratia marcescens, which tends to grow in wetter places – like your toilet bowl).

The fungus Rhodotorula mucilaginosa has two cool things going for it. One is its pinkness. The name of the genus literally translates as “rose-colored yeast” though it can also grow as yellow, orange, or red. The compounds that make these colors are carotenoids that are thought to help block out harmful ultraviolet light from damaging cells. Instead of applying that hot pink zinc sunscreen that we used to wear in the ‘80s, the fungus just decided to make its own.

Also cool is that Rhodotorula mucilaginosa can live on almost no nitrogen. All life needs nitrogen as part of its cells Some microbes – bacteria and algae – have evolved a way to “fix” nitrogen from the air, converting the gas into a form more edible for the rest of us to use. They are, in essence, air eaters.

In the 1970’s, a scientist had thought he had found – along with a path to scientific stardom – a fungus that fixes nitrogen because it grew, albeit slowly, in air that had been “scrubbed” free of its usable nitrogen. He found out he was wrong. This yeast cannot fix nitrogen, instead it does something just as amazing. It is able to filter usable nitrogen from air in which there is essentially none. In addition, it can survive with low nitrogen content in its cells. While most cells are about 10% nitrogen, Rhodotorula can survive with a cell content of only 1%. Just how this is possible is not yet clear. Perhaps symbiotic bacteria help Rhodotorula scavenge nitrogen from the air. But even if they do, that doesn’t explain how this yeast can get by with so little nitrogen in its cells given that nitrogen is in everything, from proteins to DNA.

One thing we do know is that Rhodotorula has at least one associate, namely us.

Rhodotorula is a genus that is found practically everywhere. Along with your bathroom, it has been found living in lakes (fresh-water) and oceans (salt-water). It’s found in the ice melts of the Alps and in the sand of French Mediterranean beaches. But here’s something curious, it’s more abundant in beaches highly frequented by humans.

Rhodotorula is known to cause infection in immuno-compromised individuals. Perhaps it is a regular denizen of our skin in addition to our shower curtains. I once swabbed my forehead and then wiped the swab on a petri dish, to see what would grow. After a few days with a lot of different fungi growing, I tried to isolate just one, a white shiny yeast known as the dandruff fungus, Malassezia. After the white yeast grew out in a circle to about a quarter size, a pink yeast started to speckle the surface in a ring as if it was following the growth of the white yeast. Rhodotorula? It could have been. Perhaps it was eating the Malassezia on my forehead.

It is kind of endearing to think about a pink fungus living on my head. But, tolerance only goes so far. Next time I notice that familiar pink growth in my bathroom, I will try to wipe it away clean. But don’t pity the fungus; rest assured, some cells will persist. They need so little. They will be back.

About the Author

Rachel Adams is a post-doctoral fellow in the Department of Plant & Microbial Biology at the University of California at Berkeley. She studies the dispersal of fungi in homes and tweets at @Rachel_I_Adams.