Tuesday, January 4, 2011

Serratia marcescens

If you've ever seen a red film growing in your bathtub or in your toilet, you haven't been visited by the Cat in the Hat and his pink cake; you're seeing Serratia marcescens. That also means you should scrub your bathtub/toilet right away, because Serratia marcescens can cause urinary tract infections, conjunctivitis (pink eye), and respiratory tract infections.[1] Since it actually lives off soap and shampoo residues, your best chances of eliminating the bacteria from your bathroom is to use bleach; otherwise it will just keep coming back.

Surprisingly, Serratia marcescens wasn't originally recognized as a pathogen. In fact, during the Cold War, the U.S. Navy released it in the atmosphere over the San Francisco Bay Area to simulate a biological warfare attack. They chose Serratia marcescens because it's reddish-orange pigmentation (caused by a chemical called prodigiosin, shown to the right) would make it easy to track. Much to their chagrin, this resulted in a rise in serious urinary and respiratory tract infections.[2]

One other interesting idea about Serratia marcescens: it has been proposed that its ability to grow on bread (or, more specifically, on the wafer used for the Catholic Eucharist) may have helped canonize the doctrine of transubstantiation in the Roman Catholic church. In 1263 a priest who was initially skeptical of transubstantiation claimed that he'd seen some consecrated host bleed. One year later, after investigating the claim, Pope Urban IV instituted the feast of Corpus Christi to commemorate the event.[3] Some believe that it was actually Serratia marcescens, not blood, that made the host turn red.


[1] See http://en.wikipedia.org/wiki/Serratia_marcescens#Pathogenesis. In addition to human diseases, it also causes white pox disease in elkhorn coral and aggravates flacherie disease in silkworms.

[2] See http://en.wikipedia.org/wiki/Serratia_marcescens#History.

[3] Ibid.

Image attributions:

Serratia marcescens on an agar plate is by Brudersohn, available at http://commons.wikimedia.org/wiki/File:Serratia_marcescens.jpg.

The chemical structure of prodigiosin is by Fvasconcellos, available at http://commons.wikimedia.org/wiki/File:Prodigiosin.svg.


  1. I'm quite glad you posted this. I've often wondered about the orangish stuff growing on our shower curtain (and some times bathroom ceiling) because of poor ventilation during the summer. I figured it was some kind of mold and used a cleaner with bleach. I'm so glad I did!

  2. Bleach, it's not just an anime cartoon!

    I'd wondered about that too. Usually though when my shower curtain starts to go ... off, I just replace it. It's easier that way. ;-)

  3. This is timely advice. I have it on my shower curtain. And now other people know that I'm slovenly.

  4. A few years back I would experience really bad itching all over my chest whenever I got hot. I figured that I just had dry skin, so I started taking colder showers and even tried putting on lotion. (Yuck.) But then I noticed some Serratia growing on the shower head. I washed it off and bleached it and the itching went away. I can't be sure that the two were related, but I suspect that they were.