People see pink in the bathroom and immediately think mold. It is a reasonable assumption, but the culprit is almost always a bacterium, not a fungus. The distinction matters because the treatment is different.
What Is Serratia Marcescens?
Serratia marcescens is a gram-negative bacterium found naturally in soil, water, and air worldwide. It is not something you "caught" or that your house is particularly dirty — the bacteria are in every home. They simply become visible when conditions allow a colony to grow large enough to see.
The pink color comes from prodigiosin, a pigment the bacteria produce. Interestingly, prodigiosin production is temperature-dependent. The bacteria produce the most pigment at temperatures between 77-86°F (25-30°C), which is exactly the temperature range of a warm, recently-used shower surface. At body temperature (98.6°F), prodigiosin production drops significantly, which is one reason Serratia colonies inside the body do not turn tissue pink.
Serratia marcescens was once considered completely harmless and was even used by the US military in the 1950s as a tracer organism for bioweapons testing — they sprayed it over San Francisco to study air dispersal patterns. We now know it can cause infections in immunocompromised individuals, particularly urinary tract infections and wound infections in hospital settings. For healthy people, the surface colonies in your shower are not a health risk, but it is still good practice to clean them.
Why Your Shower Specifically
The bacteria need three things to form visible colonies:
- Moisture. Constant. Your shower provides this.
- Warmth. Showers are warm during and after use, and bathrooms tend to be warmer than other rooms.
- A food source. Soap scum, shampoo residue, body oils, and mineral deposits in hard water all provide nutrients for the bacteria.
Grout lines are particularly susceptible because they are textured and porous, providing more surface area for the bacteria to colonize. Caulk lines around the tub are another favorite. You may also see it on shower curtains, around drain openings, and on shampoo bottle bottoms that sit in standing water.
The pink buildup is worse in bathrooms with poor ventilation. If your bathroom exhaust fan is not pulling air properly, moisture hangs around longer and gives the bacteria more time to grow between cleanings.
How to Clean It
Serratia marcescens colonies sit on the surface and respond well to several cleaning approaches:
Bleach solution. Mix one part chlorine bleach with 10 parts water in a spray bottle. Spray on the pink areas, let it sit for 10 minutes, then scrub with a brush and rinse. Bleach both kills the bacteria and removes the pink pigment. This is the most effective method.
Hydrogen peroxide. Spray 3% hydrogen peroxide directly on the pink areas. Let it sit for 10-15 minutes, then scrub and rinse. Less harsh than bleach and still effective.
Baking soda paste. Make a paste with baking soda and water, apply to the pink areas, scrub with a brush, and rinse. This is physically abrasive and removes the biofilm layer but does not disinfect as effectively as bleach or peroxide.
Commercial bathroom cleaners. Most tile and grout cleaners contain bleach or other biocides that will work. Look for products that specifically mention "kills bacteria" — mold-specific products also work because their active ingredients kill bacteria too.
Why It Keeps Coming Back
You clean it, it looks pristine, and two weeks later the pink is back. This is frustrating but expected. Serratia marcescens is airborne and present in your water supply in small numbers. You cannot eliminate the source. Every time moisture sits on a surface with some soap residue, the bacteria have an opportunity to colonize again.
The goal is management, not eradication. The practical approach:
- Squeegee or wipe shower surfaces after each use. This is the most impactful single habit.
- Run the exhaust fan during and for 20 minutes after every shower. Reduces ambient moisture.
- Spray shower surfaces weekly with a bleach or peroxide solution. A quick spray — no need to scrub every week. This kills nascent colonies before they become visible.
- Reduce soap residue. Rinse walls briefly after shampooing and soaping up. Soap scum feeds the bacteria.
- Fix any leaks. Chronic moisture from a dripping showerhead or a slow leak behind the wall creates a permanent bacterial habitat.
Is It Staining the Grout?
If the pink has been present for a long time, prodigiosin can stain porous grout. The stain may not fully come out with surface cleaning alone. For deep staining:
- Apply a thick paste of baking soda and bleach directly to the stained grout lines
- Cover with plastic wrap to keep it moist
- Let it sit for several hours (or overnight)
- Scrub with a stiff grout brush and rinse
If the grout is very old and porous, even this may not fully remove the stain. Applying a grout sealer after cleaning helps prevent future staining by reducing the grout's porosity.
If you are also dealing with other bathroom issues like a bathtub stopper that does not hold water or paint peeling from the ceiling, addressing the moisture problem in the bathroom will help with all of them simultaneously.
Related: Bathroom Fan Runs But Doesn't Pull Air · Why Does Paint Peel Off Bathroom Ceiling? · Bathtub Drain Stopper Won't Hold Water
Written by David Park
David writes about science and the natural world. He enjoys turning research findings into interesting, easy-to-understand articles.