It is Serratia marcescens, a common airborne bacterium, not mould and not rust. It feeds on soap and shampoo residue, it wipes off easily, and it comes back until you remove the residue it is eating rather than just the colour.
What it actually is
Serratia marcescens is a bacterium that lives more or less everywhere, arrives on air currents, and thrives on the phosphorus and fatty deposits in soap scum and shampoo. It shows up as a pink, orange or occasionally red film in shower corners, around drains, in pet bowls, and in the toilet bowl at the waterline.
People often assume it is rust from old pipes or mould. It is neither. Rust does not wipe off with a cloth; this does.
Getting rid of it
Bleach solution, one part to three, left for ten minutes, then scrubbed. In the toilet, pour half a cup of bleach into the bowl, brush it around the waterline and under the rim, leave it fifteen minutes, then flush. Ventilate the room while you do it.
The colour will be gone in one pass. The point of the exercise is the second step.
Why it keeps coming back
Killing the bacteria does nothing about its food supply. Soap scum on tile, in grout, and on the shower floor is what it is living on, and it re-colonises within days. To actually stop it:
- Squeegee or towel the shower walls after use — ninety seconds, and it beats every cleaning product
- Run the fan for twenty minutes after showering
- Switch from bar soap to a liquid body wash if you have a persistent problem; bar soap leaves far more residue
- Deep-clean the grout occasionally rather than just the tile faces
Is it dangerous?
For a healthy household, no — it is a nuisance. It is worth taking more seriously if someone in the house is immunocompromised, has an open wound, or wears contact lenses and might rinse a case in the shower. Serratia is an opportunistic pathogen and has caused infections in those situations.
None of this is a remodelling problem. If a contractor tells you pink slime means you need a new shower, find another contractor.
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