There is a particular kind of low-level domestic horror that comes from noticing a pink tinge creeping along the hem of your shower curtain. It appears gradually, almost politely, as though trying not to make a fuss – a faint blush along the bottom fold one week, a definite stain along the grout lines the next. Most people’s response follows a fairly predictable script: assume it is mould, reach for whatever cleaning spray lives under the sink, scrub at it with varying degrees of success, and feel vaguely judged by their own bathroom for the remainder of the week.
Here is the thing, though. That pink stuff is almost certainly not mould. It looks like mould, it arrives in the same damp, poorly ventilated conditions as mould, and it carries the same faint implication that you perhaps ought to be cleaning your bathroom more often. But it is an entirely different organism – and that distinction matters enormously, because it changes how you treat it, how you prevent it, and how seriously you ought to take it. By the end of this article, you will know exactly what you are dealing with, how to get rid of it properly, and – crucially – how to stop it staging a comeback the following fortnight.
First Things First – That Pink Stuff Is Not Actually Mould
Meet Serratia marcescens – Your Uninvited Bathroom Resident
The organism responsible for that distinctive pink-to-salmon colouring is Serratia marcescens, and it is a bacterium rather than a fungus. This is not a pedantic distinction. It has real practical consequences for how you approach cleaning it, because products formulated to kill mould – which target fungal cell structures – are largely ineffective against a bacterial colony. Spraying antifungal bathroom cleaner on Serratia marcescens is a bit like trying to fix a leaking pipe with a tin of paint. You will feel like you are doing something, but the underlying problem remains entirely unimpressed.
Serratia marcescens produces a pigment called prodigiosin, which is responsible for the pink-to-reddish colouring you see on surfaces. It is airborne, meaning it arrives in your bathroom entirely uninvited and without any implication of poor housekeeping – it is simply present in the environment and settles wherever conditions suit it. And the conditions it loves most are warm, moist environments with a readily available food source. A London bathroom ticks every box: warmth from regular hot showers, persistent humidity, and an all-you-can-eat buffet of soap scum, body oils, shampoo residue, and the mineral deposits left behind by our legendarily hard tap water. A Victorian terrace bathroom with no extractor fan in February? Practically a five-star resort.
Is It Actually Dangerous?
The honest answer is: for most healthy adults, not especially – but not nothing either. Serratia marcescens is what microbiologists classify as an opportunistic pathogen. In a fully healthy person with a functioning immune system, bathroom exposure is unlikely to cause anything beyond the mild skin and eye irritation that contact with any bacterial colony might produce. It is not cause for panic.
However, it is worth taking seriously if anyone in your household is immunocompromised, elderly, very young, pregnant, or managing a chronic illness. In vulnerable individuals, Serratia marcescens has a well-documented history of causing urinary tract infections, respiratory infections, and in clinical settings – where it has caused more than its share of trouble historically – more serious complications. It is also worth noting that it can survive on surfaces for longer than many people assume, and that the same soap scum feeding it in your shower can also harbour it on bath toys, toothbrush holders, and other surfaces that come into closer contact with people.
The practical takeaway: treat it promptly, clean it thoroughly, and prevent it from re-establishing rather than aesthetically tolerating it.
Removing Pink Residue from Shower Curtains
Fabric and Plastic Curtains – Know Your Material First
Shower curtains broadly fall into two camps – fabric (cotton, linen, or polyester blends) and plastic or PEVA – and they need to be treated differently, so it is worth establishing what you are working with before reaching for anything.
For fabric curtains, the washing machine is your best friend, provided you use it correctly. Remove the curtain from its rings, check the care label, and wash it on the hottest cycle the fabric will tolerate. Add your usual detergent, but also add half a cup of white vinegar to the fabric softener compartment and, if the curtain is white or light-coloured, a scoop of oxygen-based bleach. Oxygen bleach – the kind marketed as colour-safe or fabric-safe – is effective against bacteria without the harshness of chlorine bleach, which can weaken fabric fibres and degrade curtain liners over time. A useful trick: throw two or three old towels into the drum alongside the curtain. They add mechanical agitation that genuinely improves the scrubbing action, and your curtain will come out considerably cleaner for it. Air dry rather than tumble drying, and rehang promptly to prevent new creases from setting and giving the bacterium fresh folds to hide in.
Plastic and PEVA Curtains – The Hands-On Approach
Plastic and PEVA curtains require a more manual approach, and they also tend to be the first candidates for the bin when a good clean would actually restore them completely. Before replacing it, give it a proper chance.
Take the curtain down and lay it flat in the bath. Mix a solution of warm water, a good squeeze of washing-up liquid, and either white vinegar (equal parts with the water) or a diluted hydrogen peroxide solution – around one part 3% hydrogen peroxide to two parts water. Apply the solution liberally to the affected areas and leave it to work for ten to fifteen minutes. Then scrub with a stiff-bristled brush, paying particular attention to the hem and any folds where residue collects and moisture lingers. Rinse thoroughly with warm water and hang to dry immediately.
Replacement is warranted when the pink staining has penetrated deep into the material and resists repeated treatment, or when the curtain itself has begun to crack, yellow, or become structurally degraded. At that point, you are cleaning the curtain largely out of stubbornness, and a new one is the pragmatic call.
Tackling Pink Residue in Grout Lines
Why Grout Is Such a Perfect Habitat
Grout lines are arguably the ideal environment for Serratia marcescens, and understanding why makes it easier to tackle them effectively. Grout is porous, meaning it absorbs both moisture and the organic residue that feeds bacterial growth. It sits at the junction between tiles, precisely where water pools, soap scum accumulates, and airflow is minimal. And in most domestic bathrooms, it is genuinely difficult to dry thoroughly – even the most conscientious post-shower squeegee leaves grout lines damp for hours.
For a basic grout treatment, make a thick paste of bicarbonate of soda and water – you want a consistency similar to toothpaste – and apply it directly to the affected grout lines with an old toothbrush or a dedicated grout brush. Leave it for ten to fifteen minutes, then scrub firmly along the lines and rinse with warm water. For more stubborn cases, hydrogen peroxide solution applied directly to the grout is an effective escalation – spray it on, allow five to ten minutes of contact time, and scrub. If the colonisation is heavy and the above approaches are not shifting it, a diluted bleach solution (one part bleach to four parts water, good ventilation, rubber gloves, no mixing with anything else) is the appropriate next step. Rinse thoroughly after any bleach application and allow to dry completely before the next shower.
Coloured Grout and Epoxy Grout – Proceed with Extra Caution
Coloured grout introduces a complication that catches a good number of people out. Bleach-based treatments can permanently strip the pigment from coloured grout, transforming a pink-stained grout line into a bleached-white one – which is arguably more visually jarring than the original problem and is, unfortunately, irreversible. If your grout is anything other than white or near-white, treat with caution.
For coloured grout, start with the bicarbonate paste and hydrogen peroxide approach before even considering bleach. Crucially, always do a patch test in an inconspicuous corner – behind the toilet, along the lowest row of tiles – and leave it for the full treatment time before assessing the result. If there is any colour change, stop and rethink your product choice.
If you are considering re-grouting at any point, it is worth knowing that epoxy grout is a genuinely superior option for bathroom environments. Unlike cement-based grout, epoxy grout is non-porous, does not absorb moisture or soap residue, and is dramatically more resistant to bacterial colonisation. It costs more and is more demanding to apply, but the long-term maintenance reduction is considerable.
Prevention – Because You Really Do Not Want to Be Doing This Every Fortnight
The Environmental Conditions You Need to Change
Serratia marcescens is not a sign of a dirty bathroom – but it is a fairly direct readout of a damp one. The single most impactful change most London bathrooms need is improved ventilation. If your extractor fan is more decorative than functional, or if your bathroom has no fan at all (a surprisingly common situation in older converted flats), that is the root issue that no amount of cleaning product will compensate for. Run the extractor fan during every shower and for at least twenty minutes afterwards. If natural ventilation is your only option, open a window post-shower even in winter – the temperature drop is considerably less harmful than the persistent humidity.
Get into the habit of running a squeegee over tiles and the shower curtain after every use. It takes approximately forty-five seconds and removes the moisture film that the bacterium depends on. Keep a small spray bottle of diluted white vinegar or hydrogen peroxide solution in the shower, and give grout lines and the curtain hem a quick weekly spritz as a preventative measure. You are not cleaning at that point – you are maintaining a hostile environment for something that would very much like to move back in.
Switching Up Your Products and Habits
A less obvious but genuinely effective preventative measure is switching from bar soap to liquid body wash. Bar soap leaves a significantly higher volume of fatty residue on shower surfaces – the familiar grey-white scum that Serratia marcescens treats as a perfectly balanced meal. Liquid products produce far less of it, which meaningfully reduces the available food source.
After every shower, take five seconds to rinse the curtain down with clean water from the showerhead, flushing residual soap from the surface before it can settle and feed anything. Do a targeted clean of grout lines once a month rather than waiting for visible growth to appear – at that stage, you are removing an established colony rather than preventing one from forming, and the latter is considerably less work. The difference in effort between regular light maintenance and an occasional intensive clean is enormous, and your bathroom will reflect it.