S. marcescens: Infections, Causes, and Prevention

Serratia marcescens is a rod-shaped, gram-negative bacterium best known for two things: leaving pink or reddish stains on damp household surfaces and causing stubborn hospital-acquired infections that resist many standard antibiotics and disinfectants. It thrives in moist environments, from soil and water to shower tiles and sink drains, and while it rarely troubles healthy people outside hospitals, it is a serious opportunistic pathogen in intensive care units, especially among newborns, immunocompromised patients, and people with invasive medical devices. Understanding where this organism hides, how it overwhelms hospital defenses, and what can actually keep it in check matters for patients, caregivers, and anyone who has ever wondered about that pink ring in the toilet bowl.

Where S. marcescens Shows Up in the Wild

S. marcescens is remarkably versatile. It has been isolated from soil, freshwater, plant surfaces, and the guts of insects. In the plant world it can act as a beneficial microbe, colonizing roots and promoting growth by breaking down chitin in the soil and making nutrients more available to crops.¹ Some strains produce chitinases so effectively that they have been studied as biological pest-control agents, showing larvicidal activity against several insect species and nematodes in laboratory settings.² In marine ecosystems, the bacterium has been identified as the cause of white pox disease, a lethal infection of the Caribbean elkhorn coral Acropora palmata.³

At home, you are most likely to encounter S. marcescens as a pink or orange-pink film on shower curtains, toilet bowls, tile grout, and anywhere else that stays consistently damp. The bacterium does not come from the water supply itself; it is airborne or carried on skin and thrives wherever standing moisture and trace nutrients from soap residue give it a foothold.⁴ The pink color comes from prodigiosin, a pigment the organism produces more readily at lower temperatures. At human body temperature, around 37 °C, the gene cluster responsible for prodigiosin is largely switched off at the level of gene transcription, which is why clinical isolates from infected patients often appear colorless.⁵ Pigmented cells do accumulate energy faster and multiply more quickly during dense growth, which may help the organism dominate wet surfaces in the cooler conditions of a bathroom.⁶

Who Gets Infected and How

For healthy adults living at home, S. marcescens is almost never a threat. The infections it causes are overwhelmingly hospital-acquired, and certain patient groups carry most of the risk. Neonates in intensive care units are particularly vulnerable. In one outbreak investigation at a tertiary hospital in Mexico, 93% of the affected newborns had an invasive medical device such as a vascular catheter before infection developed.⁷ In NICU settings, colonized or infected newborns themselves become the main reservoir, particularly in the respiratory and gastrointestinal tracts, and transmission typically happens via the hands of healthcare workers.⁸

Adults in ICUs face a similar pattern. An analysis of carbapenem-resistant S. marcescens infections found that neonates and adults each accounted for about half of cases, with the respiratory tract as the most frequent site of infection.⁹ Beyond the lungs, the bacterium causes urinary tract infections, wound infections, bloodstream infections, and meningitis in hospitalized patients.

Outside the hospital, one well-documented risk group is people who inject drugs. A study of Serratia bloodstream infections found that the most common complications were endocarditis (about 12% of all cases) and bone infection (about 10%), and patients who used IV drugs had a dramatically higher complication rate, with over half developing complications compared to fewer than one in ten among non-users.¹⁰ This connection was recognized as far back as the 1970s, when S. marcescens accounted for 14% of all endocarditis cases among IV drug users in the San Francisco Bay Area.¹¹

Contact Lens Infections

S. marcescens keratitis, an infection of the cornea, is rare but can be sight-threatening. It occurs primarily in contact lens wearers and is often linked to contaminated lens storage solutions.¹² The bacterium has a notable ability to survive and even grow in certain chlorhexidine-based disinfecting solutions intended for rigid gas-permeable lenses. In laboratory tests, 11 of 12 S. marcescens strains demonstrated the ability to grow in those solutions, with adapted cells resuming multiplication within days of being placed in the disinfectant.¹³ Even among modern multipurpose solutions, testing against bacterial biofilms found that only five out of 14 solutions produced a meaningful reduction in S. marcescens biofilm.¹⁴

For contact lens wearers, the practical takeaway is that proper hygiene with lenses matters more than the brand of solution. Rinsing and air-drying the lens case, never topping off old solution, and replacing cases frequently all reduce the chances of bacterial colonization. Sleeping in lenses or swimming with them in creates the kind of warm, moist, low-oxygen environment that S. marcescens exploits.

What Makes It So Hard to Kill

Two traits make S. marcescens especially troublesome in clinical settings: its intrinsic resistance to many antibiotics and its ability to survive in disinfectants that should kill it.

On the antibiotic side, S. marcescens naturally carries a chromosomal gene for an enzyme called AmpC beta-lactamase, which breaks down many penicillins and older cephalosporins. Laboratory experiments have shown that through stepwise selection, mutants can develop high-level resistance to even advanced cephalosporins.¹⁵ More alarming is the emergence of carbapenem-resistant strains. Carbapenems are the antibiotics clinicians typically turn to when common drugs fail, and resistance to them leaves very few options. A bloodstream-infection isolate from China was found to carry resistance genes for multiple carbapenems on mobile genetic elements that could transfer to other bacterial species at low fitness cost, meaning the receiving bacteria did not grow slower for carrying the resistance.¹⁶ A Swiss hospital saw outbreaks involving both S. marcescens and Klebsiella pneumoniae sharing carbapenem-resistance plasmids between them.¹⁷ In a Japanese survey of over 5,000 clinical isolates, about 0.5% were carbapenem-resistant, and many of those were resistant to broad swaths of other drug classes as well, including fluoroquinolones and aminoglycosides.¹⁸

Disinfectant resistance may be even more surprising. In a classic hospital investigation, S. marcescens was found living in 2% chlorhexidine hand-washing solution, surviving in concentrations far above what should have killed it, and remaining viable in the fluid for over two years.¹⁹ In another ICU investigation, antiseptic handwashing soap containing 1% triclosan was found contaminated with the organism.²⁰ A more recent outbreak in a Hungarian adult ICU was traced to a quaternary ammonium compound disinfectant that the bacterium had overcome. The hospital had to switch to a different disinfectant with a shorter required contact time and entirely replace its cleaning equipment before the outbreak stopped.²¹

Biofilm and Virulence

A major reason S. marcescens persists on hospital surfaces and inside patients is its capacity to form biofilms. Biofilm is essentially a slimy, structured community of bacterial cells embedded in a self-produced matrix that sticks to surfaces, whether those are sink drains, catheter tubing, or lung tissue. Inside this matrix, bacteria are shielded from both the immune system and antibiotics, tolerating drug concentrations that would readily kill free-floating cells. In S. marcescens, biofilm formation is coordinated through chemical signaling systems that also regulate the production of enzymes, toxins, and motility.²²

Beyond biofilm, the bacterium deploys a toolkit of virulence factors. It produces hemolysins that damage red blood cells, proteases that break down host tissues, and siderophores that scavenge iron from the host to fuel bacterial growth.²³ One protease, serralysin, triggers inflammatory responses in host tissues, contributing to the tissue damage seen during active infection.²⁴ Some clinical strains also carry sugar-based capsules on their surface that help them dodge the immune system. Research on different capsule types has found that strains with certain sialylated capsules can resist being engulfed by macrophages, a key front-line immune cell. Only specific capsule lineages showed this ability, which may help explain why some strains cause more severe infections than others.²⁵

How Hospital Outbreaks Are Stopped

Because the specific source of a S. marcescens outbreak often cannot be pinpointed to a single contaminated item, containment relies on layered interventions applied quickly. A NICU outbreak report from Europe showed that strict hand hygiene enforcement, enhanced surface cleaning, patient screening to find colonized but asymptomatic infants, contact precautions, and staff education together contained the outbreak within three weeks.²⁶ A similar outbreak in a Singapore neonatal unit was brought under control using cohorting of positive cases (keeping infected or colonized babies together and cared for by dedicated staff), carriage screening, enhanced environmental cleaning, and emphasis on alcohol-based hand rubs.²⁷

One often-overlooked source is hospital plumbing. Sink drains and water taps can harbor the organism and serve as a silent reservoir that re-seeds the environment even after surfaces are cleaned. Molecular typing tools have confirmed this link. A genetic fingerprinting method applied to a NICU found that the strain responsible for an outbreak in the unit matched the strain recovered from a sink drain, confirming the drain as a reservoir.²⁸ These genomic surveillance techniques, which compare bacterial DNA across patient and environmental isolates, have become increasingly important for tracking how the bacterium spreads through a ward and verifying that control measures worked.²⁹

The lesson from decades of outbreak reports is that no single measure works alone. Hand hygiene is essential but insufficient when the disinfectant itself is contaminated, or when drains keep reintroducing the organism. Successful containment almost always requires a bundle approach: better hand hygiene, switching or verifying disinfectants, screening patients, cohorting positive cases, and sometimes replacing contaminated infrastructure like cleaning equipment or plumbing components.

The Military Experiment History

Part of the reason S. marcescens was once so poorly understood as a pathogen is that for much of the twentieth century, it was considered harmless. Its vivid red pigment made it easy to track, and because it was deemed non-pathogenic, the U.S. military used it in biological warfare simulation experiments during the Cold War, releasing the organism over populated areas to study how airborne agents would disperse. Members of the public and military personnel were unknowingly exposed. When the experiments came to light through press reports in the 1970s, they led to U.S. congressional hearings.³⁰ The episode is a stark reminder that labeling an organism “non-pathogenic” based on limited evidence can have consequences, and it contributed to the eventual recognition that S. marcescens was, in fact, capable of causing serious disease in vulnerable people.

Experimental Approaches to Fighting Biofilm

Because biofilm is central to how S. marcescens persists in hospitals and resists treatment, researchers are exploring strategies that target biofilm formation rather than just trying to kill individual cells. One approach involves disrupting the chemical signaling that bacteria use to coordinate biofilm construction. In laboratory experiments, glyceryl trinitrate (a compound better known in cardiology as nitroglycerin) reduced S. marcescens biofilm formation by roughly 88% at sub-lethal concentrations, while also suppressing protease production and motility.³¹

Bacteriophages, viruses that specifically infect bacteria, offer another avenue. A phage isolated specifically against S. marcescens was found not only to kill the bacterium but also to reduce biofilm formation in E. coli strains it did not even infect, suggesting it could destabilize mixed-species biofilms on hospital surfaces.³² Another study found that when S. marcescens is infected by a particular phage, the bacterium responds by massively overproducing prodigiosin, which has shown anticancer and immune-modulating properties. Researchers have suggested this response could potentially be exploited for pharmaceutical production of prodigiosin.²³ These approaches are still in the laboratory stage, but as multidrug resistance narrows the usefulness of conventional antibiotics, interest in anti-biofilm and phage-based therapies is growing steadily.

Everyday Prevention at Home

For the average person, the main interaction with S. marcescens is that pink film on bathroom surfaces. It is not dangerous to healthy individuals, but it is persistent. Regular cleaning with a bleach-based solution or hydrogen peroxide, followed by thorough drying of surfaces, keeps it at bay. The bacterium feeds on fatty residues left by soap and shampoo, so rinsing down shower walls after use and keeping surfaces dry removes both its food source and the moisture it needs. Ventilation matters too: running an exhaust fan during and after showers reduces the humidity that allows S. marcescens to flourish.

For contact lens users, the evidence suggests that the lens storage case is the critical weak point. Emptying the case, rubbing it with clean fingers, rinsing with fresh solution (never water), and air-drying it face down after each use disrupts the biofilm cycle. Replacing the case at least every three months is a simple step that reduces the buildup of bacteria that multipurpose solutions alone may not eliminate. If you ever notice a pinkish tint in your lens case or on your lenses, that is a signal to discard the case and solution immediately and consider seeing an eye care provider, especially if you have any redness or discomfort in your eyes.