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I was wondering if anybody had studied or was knowledgeable about the pink growth that people often find around water fixtures in their homes, especially bath tubs and showers?

My understanding is that this pink growth is actually a bacteria and not a fungus as many assume (source1,source2). Supposedly, the bacteria, Serratia marcescens is airborne and thrives in areas of retained moisture and fatty residue.

My main questions are:

  1. How ubiquitous is this bacteria? What is the bacteria's geographical range of occurrence?
    • For example, is this species more prevalent in more humid environments?
  2. Are there other species of Serratia that can create similar pink biofilms in tubs? (Are the sources of "pink" separated by geographic location?)
    • Is S. marcescens even forming a biofilm in these pink stains??
  3. What likely introduces S. marcescens into a bathroom?

I ask, because I've seen this bacteria both present or absent from similarly clean or dirty bathrooms, and thus far have not really seen any consistent patterns in the bacteria's spatial occurrence.

Any other interesting facts or sources on S. marcescens would be great!

enter image description here

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    $\begingroup$ I think that it may be well be inorganic calcium-deposits, or other salts. Tap water is often rich in calcium-salts and the reddish color I've seen pretty often down under too. I don't know where you are located, but in Oz there's a lot of iron in the ground, perhaps explaining the reddish hue. All guesses though. $\endgroup$
    – AliceD
    Jan 11, 2016 at 22:08
  • $\begingroup$ Is the film slimy ? Try checking with a swab $\endgroup$
    – WYSIWYG
    Jan 12, 2016 at 13:03
  • $\begingroup$ @WYSIWYG Sometimes but not always. I don't have this in my own home, so I don't get to check too often... $\endgroup$ Jan 15, 2016 at 19:06
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    $\begingroup$ Also @Christiaan: the deposits I'm talking about are more pink and therefore indicative of S. marcescens. Iron deposits would be more dark reddish (or reddish yellow if also associated with heightened aluminum). Calcium deposits would typically be white. $\endgroup$ Jan 19, 2016 at 4:12
  • $\begingroup$ very difficult to say in this-way without help of microscope $\endgroup$ Aug 9, 2016 at 13:53

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I live in St. Louis, MO and get this in my bathtub over time, but not in my other sinks. I actually think @Christiaan is correct here, but with a few additional observations:

  1. I don't think it's bacteria. When dry, it's not slimy, nor does it grow back quickly when wiped away. It feels much more like a mineral deposit.
  2. It was no longer pink when using a different shampoo. Upon returning to an older brand, the color returned to pink.
  3. It's only directly around the drain, it isn't on the walls or anywhere else.

These lead me to think it's a mix of mineral deposits from the hard water and potential reactions with certain soaps/shampoos, at least in my case. These are only based on my own observations though, there very well could be a pink bacteria/fungi that tends to inhabit damp areas.

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    $\begingroup$ No it's definitely bacteria. They like to grow wherever water pools with organic matter or certain soap deposits. You typically have water around your drain long enough for the chlorine or other treatment chemicals to evaporate off to allow the bacteria to live. Coming or going with different soaps just means that one of the soaps had more of the ingredients (fats and /or phosphorous) they prefer $\endgroup$ Jan 19, 2016 at 4:05
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    $\begingroup$ Well, I guess at least you know that geographically it's present in the midwest as well. For what it's worth, I also live in a basement apartment, so it's likely more humid than most. $\endgroup$ Jan 19, 2016 at 14:45
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It could be Haloarchaea

  • so @theforestecologist not Bacteria but Archaea!

enter image description here

https://en.wikipedia.org/wiki/Haloarchaea

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    $\begingroup$ Thanks for the post, but this is very unlikely since salt water is not used in plumbing. Your proposed organism is halophilic, meaning it likes to live in salty conditions! $\endgroup$ Feb 13, 2021 at 19:25

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