Serratia marcescens is one of the more stubbornly resistant bacteria that clinicians encounter in hospital settings. It carries built-in defenses against several common antibiotic classes, including polymyxins and many older penicillins, and it readily picks up new resistance genes from other bacteria. Treatment typically centers on carbapenems, certain cephalosporins, and aminoglycosides, but even those options are narrowing as multidrug-resistant strains spread through intensive care units worldwide. Understanding why this organism is so difficult to kill, and what still works against it, matters for anyone dealing with a Serratia infection or trying to prevent one.
Why Serratia marcescens Starts Out Resistant
Unlike bacteria that begin susceptible and gradually acquire resistance through exposure, S. marcescens comes equipped with several resistance mechanisms from the start. These are encoded in its own chromosome, meaning every strain carries them regardless of antibiotic history. Clinical isolates are frequently described as multidrug resistant, a consequence of the combined activity of intrinsic, acquired, and adaptive resistance elements working together.1PubMed Central. Serratia marcescens antibiotic resistance mechanisms of an opportunistic pathogen: a literature review
One of the most clinically relevant intrinsic defenses is the AmpC beta-lactamase, an enzyme that breaks down certain beta-lactam antibiotics. Many Enterobacterales carry a version of AmpC, but the way it behaves in S. marcescens has some quirks. In related bacteria like Enterobacter cloacae, a single mutation in a gene called ampD can unleash high-level AmpC production, leading to strong resistance against drugs like ceftriaxone. In S. marcescens, mutations in two separate genes are needed before AmpC production ramps up, and even then, the resulting resistance level is much lower. Researchers found that when E. cloacae lost its ampD gene, the ceftriaxone concentration needed to stop growth shot up to 96 micrograms per milliliter; for S. marcescens with both genes deleted, it stayed around 1 microgram per milliliter.2PubMed Central. Divergent genetic landscapes drive lower levels of AmpC induction and stable de-repression in Serratia marcescens compared to Enterobacter cloacae That sounds like good news, but during antibiotic treatment, the enzyme can still be induced to higher levels, catching clinicians off guard.
Another built-in defense is natural resistance to polymyxins, including colistin, which is often considered a last-resort drug for other Gram-negative infections. This means one of the final fallback options for many resistant bacteria is simply not on the table for Serratia.
Efflux Pumps That Flush Drugs Out
Even when an antibiotic manages to cross the bacterial cell membrane, S. marcescens has molecular pumps that actively push it back out. The best-characterized of these is SdeXY, a pump belonging to a large family of transport proteins. When researchers deleted the genes for SdeXY in a laboratory strain, the bacteria became dramatically more sensitive to a broad spectrum of drugs, confirming that this pump is a major contributor to intrinsic multidrug resistance.3Scientific Reports. Comprehensive analysis of resistance-nodulation-cell division superfamily (RND) efflux pumps from Serratia marcescens, Db10 SdeXY was the first pump of its type characterized in multidrug-resistant S. marcescens.4Journal of Antimicrobial Chemotherapy. An RND-type multidrug efflux pump SdeXY from Serratia marcescens
A second pump system, SdeAB, contributes to resistance against fluoroquinolones, a drug class commonly used for urinary and respiratory infections. Gene expression studies of clinical isolates have confirmed that upregulation of SdeAB is one of the major mechanisms these bacteria use to resist fluoroquinolones.5Indian Journal of Medical Microbiology. Gene expression analysis of the SdeAB multidrug efflux pump in antibiotic-resistant clinical isolates of Serratia marcescens Together, these pump systems mean that even drugs capable of entering the cell may not stay there long enough to work.
Resistance Genes Borrowed from Other Bacteria
Beyond what it carries natively, S. marcescens picks up resistance genes from the bacterial world around it, usually via plasmids, small circles of DNA that can hop between species. This acquired resistance is where things get especially worrying, because the genes involved can neutralize some of the most powerful antibiotics available.
Extended-spectrum beta-lactamases, or ESBLs, are enzymes that break down advanced cephalosporins. Plasmid-borne ESBL genes have been found in nosocomial S. marcescens populations, including rare variants like blaTEM-61 carried on transferable plasmids that enable spread across bacterial populations within a hospital.6PubMed. Genomic characterisation and epidemiology of nosocomial Serratia marcescens isolates resistant to ceftazidime and their plasmids mediating rare bla(TEM-61) Other novel ESBL genes, such as blaTLA-3, have been discovered on the same plasmids that carry new aminoglycoside resistance genes, packing multiple resistance weapons onto a single transferable element.7PubMed. New plasmid-mediated aminoglycoside 6′-N-acetyltransferase, AAC(6′)-Ian, and ESBL, TLA-3, from a Serratia marcescens clinical isolate
Carbapenemases are perhaps the most alarming acquired resistance genes, because carbapenems are one of the go-to treatments for serious Serratia infections. Chromosomally encoded SME carbapenemases were first identified in the UK in 1982 and have since appeared in the US, Canada, Argentina, and Switzerland.8Journal of Antimicrobial Chemotherapy. Serratia marcescens producing SME carbapenemases: an emerging resistance problem in the UK? In the US, carbapenem-resistant strains producing SME-1 have been detected through national surveillance programs, though carbapenem resistance via these enzymes remains relatively rare overall.9PubMed. Carbapenem-resistant Serratia marcescens isolates producing Bush group 2f beta-lactamase (SME-1) in the United States: results from the MYSTIC Programme
More recently, strains have appeared that stack multiple carbapenemase genes, creating what amounts to layered armor against carbapenems. A case in China documented the first bloodstream infection caused by a strain co-producing NDM-1, KPC-2, and the chromosomal SRT-2 enzyme, with the carbapenemase genes carried on separate plasmids alongside an aminoglycoside resistance gene.10PubMed Central. Emergence of carbapenem-resistant Serratia marcescens co-harboring blaNDM-1, blaKPC-2, and blaSRT-2 in bloodstream infection Strains like this leave very few treatment options.
Aminoglycoside resistance in S. marcescens has its own disturbing trajectory. A plasmid-borne gene called rmtB, encoding an enzyme that chemically modifies the bacterial ribosome to prevent aminoglycosides from binding, was found in S. marcescens clinical isolates. This type of resistance mechanism had previously been confined to the soil organisms that naturally produce aminoglycosides, so its jump into hospital pathogens was a significant development.11PubMed Central. Plasmid-mediated 16S rRNA methylase in Serratia marcescens conferring high-level resistance to aminoglycosides
Biofilms and Why Surface Infections Persist
S. marcescens readily forms biofilms, structured communities of bacteria encased in a slimy matrix of sugars, proteins, and DNA. Biofilms are a major reason catheter-associated infections keep coming back: bacteria inside the matrix are physically shielded from antibiotics and immune cells. One study tested antibiotics that worked perfectly well against free-floating S. marcescens cells, including ceftriaxone, kanamycin, and gentamicin, and found that only chloramphenicol managed to reduce both biofilm mass and the viability of bacteria within it.12PubMed Central. Killing of Serratia marcescens biofilms with chloramphenicol That disconnect between lab susceptibility testing, which uses free-floating bacteria, and real-world biofilm behavior is a persistent clinical challenge.
The structural backbone of Serratia biofilms includes extracellular DNA, which acts as a kind of scaffold. Research into plant-derived phenolic compounds found that coumaric acid and syringic acid could inhibit biofilm formation by roughly 70–80% and disrupt mature biofilms by degrading this extracellular DNA. Microscopy confirmed that treated biofilms showed disrupted architecture and mostly dead cells.13PubMed Central. Deciphering the mechanisms of antibacterial and antibiofilm potential of phenolic compounds against Serratia marcescens These findings are still in the laboratory stage, but they illustrate a potential avenue for adjunct therapies that weaken the biofilm so that conventional antibiotics can reach the bacteria inside.
Quorum Sensing and the Coordination of Virulence
S. marcescens uses a chemical communication system called quorum sensing to coordinate group behaviors. When enough bacteria are present, signaling molecules trigger the simultaneous production of pigments, enzymes that damage host tissue, swarming motility, and the biofilm matrix itself. Mutants unable to produce these signaling molecules form only thin, undeveloped biofilms lacking normal cell aggregates and differentiated structures.14FEMS Microbiology Reviews. Quorum sensing in Serratia
This communication system is an appealing drug target because disrupting it does not kill the bacteria outright, reducing the selective pressure that drives resistance. Several compounds have shown promise in laboratory studies. Eugenol, found in clove oil, significantly reduced biofilm formation, pigment production, and the expression of genes controlling swarming and surface attachment in S. marcescens.15PubMed Central. Quorum sensing-regulated functions of Serratia marcescens are reduced by eugenol Petroselinic acid inhibited quorum-sensing-dependent virulence factors including protease and exopolysaccharide production, along with swimming and swarming motility, without affecting bacterial growth itself.16PubMed. Inhibition of quorum sensing-dependent biofilm and virulence genes expression in environmental pathogen Serratia marcescens by petroselinic acid Glyceryl trinitrate, better known as a heart medication, inhibited biofilm formation by about 88% and prodigiosin pigment production by about 83% at sub-inhibitory concentrations.17PubMed Central. Silencing the nosocomial pathogen Serratia marcescens by glyceryl trinitrate None of these are ready for clinical use against Serratia infections, but the idea of disarming the bacteria rather than trying to kill them outright is gaining traction.
What Works in the Clinic Right Now
A systematic review of antimicrobial treatment for invasive S. marcescens infections concluded that the treatment backbone should include carbapenems or aminoglycosides in combination with third-generation cephalosporins, and potentially fourth-generation cephalosporins. For uncomplicated urinary tract infections, trimethoprim-sulfamethoxazole (cotrimoxazole) was considered a reasonable option.18PubMed Central. Antimicrobial Treatment of Serratia marcescens Invasive Infections: Systematic Review In practice, treatment is always guided by susceptibility testing of the specific isolate, because resistance profiles vary widely between strains.
A key clinical dilemma arises when S. marcescens tests non-susceptible to third-generation cephalosporins, which often signals AmpC overproduction. In that situation, carbapenems have traditionally been the default. But a retrospective analysis found that carbapenem-sparing regimens were not associated with higher 14-day mortality compared with carbapenems, even among ICU patients. The catch is that isolates non-susceptible to third-generation cephalosporins also showed high rates of co-resistance to common alternatives: about 57% were non-susceptible to cefepime, roughly 59% to levofloxacin, about 53% to tigecycline, and 40% to trimethoprim-sulfamethoxazole.19Journal of Microbiology, Immunology and Infection. Outcomes associated with carbapenem-sparing versus carbapenem-containing therapy for third-generation cephalosporin non-susceptible Serratia marcescens bacteremia: a retrospective cohort analysis So while sparing carbapenems may be possible in selected cases, the co-resistance rates mean that alternatives need to be chosen carefully with lab guidance.
For carbapenem-resistant strains that do not produce metallo-beta-lactamases, newer beta-lactam/inhibitor combinations offer hope. In a laboratory evaluation, about 98% of such isolates were susceptible to ceftazidime-avibactam, and about 71% were susceptible to meropenem-vaborbactam.20The Brazilian Journal of Infectious Diseases. Evaluation of the minimum inhibitory concentration of meropenem and new therapeutic options for Serratia marcescens and Morganellaceae Enterobacterales with reduced susceptibility to meropenem These combinations pair a beta-lactam antibiotic with a compound that blocks the resistance enzyme, effectively restoring the drug’s activity. They represent one of the more promising developments for treating otherwise untreatable strains.
Resistance Can Emerge During Treatment
One of the more unsettling aspects of S. marcescens is that resistance can develop while a patient is being treated. A well-documented case tracked three sequential isolates from the same patient and found that by the third isolate, total beta-lactamase activity had increased 128-fold compared with the first. Both AmpC and the SME-1 carbapenemase were massively overproduced. Genetic analysis traced this to a handful of mutations, including a change in the ampD gene that de-repressed AmpC production.21Journal of Antimicrobial Chemotherapy. Selection of hyperproduction of AmpC and SME-1 in a carbapenem-resistant Serratia marcescens isolate during antibiotic therapy This is why repeat susceptibility testing during prolonged Serratia infections can be so important: an antibiotic that worked at the start of treatment may stop working partway through.
Hospital Outbreaks and Environmental Persistence
S. marcescens has an unusual talent for surviving in hospital environments, including in solutions meant to kill it. During one outbreak, investigators discovered that the bacteria had contaminated a 2% chlorhexidine hand-washing solution. Electron microscopy showed the organisms embedded in a fibrous matrix on the walls of storage bottles, and viable bacteria could be recovered from the solution after 27 months of storage. The strains could survive in chlorhexidine concentrations up to 20,000 micrograms per milliliter, far above what is normally used.22PubMed Central. Prolonged survival of Serratia marcescens in chlorhexidine A 2022 outbreak in a Hungarian ICU caused severe bloodstream infections linked to disinfectant-resistant S. marcescens.23PubMed Central. Nosocomial outbreak caused by disinfectant-resistant Serratia marcescens in an adult intensive care unit, Hungary, February to March 2022
These episodes highlight that infection control for Serratia goes beyond choosing the right antibiotic. If the organism survives in hand-wash dispensers, intravenous fluid lines, or other wet surfaces, antibiotics alone will not stop it spreading. Hospital infection control teams typically investigate shared liquid reservoirs, soap dispensers, and tap water as potential sources during outbreaks.
How Serratia Hides Inside Human Cells
An under-appreciated aspect of S. marcescens biology is its ability to invade and survive inside human cells that are not professional immune cells. Research has shown that after entering non-immune cells, the bacteria multiply inside large membrane-bound compartments. These compartments initially look like autophagosomes, the structures cells normally use to digest invaders, but Serratia prevents them from maturing into the acidic, destructive form that would kill the bacteria. The vacuoles remain non-acidic with no degradative properties, creating what amounts to a safe house.24PloS one. Serratia marcescens is able to survive and proliferate in autophagic-like vacuoles inside non-phagocytic cells This intracellular survival may help explain why some Serratia infections relapse after apparently successful antibiotic treatment: bacteria sheltered inside host cells are shielded from drugs that do not penetrate well into those compartments.
Phage Therapy as an Emerging Alternative
With conventional antibiotics losing ground, some researchers are turning to bacteriophages, viruses that specifically infect and kill bacteria. For S. marcescens, this approach is still in its infancy, but early results are encouraging. Researchers identified a phage called Spe5P4 whose genome was confirmed to be free of virulence genes, lysogeny-related genes, and antibiotic-resistance genes, making it a potentially safe candidate for therapeutic use against multidrug-resistant S. marcescens causing pulmonary infections.25PubMed Central. Exploration of the feasibility of clinical application of phage treatment for multidrug-resistant Serratia marcescens -induced pulmonary infection Phage therapy faces substantial regulatory and practical hurdles, including the need to match specific phages to specific bacterial strains and the bacteria’s potential to evolve phage resistance. But as a complement to antibiotics rather than a replacement, phages represent one of the more plausible new tools in the pipeline.
Genetic Diversity and the Challenge of Surveillance
Tracking S. marcescens resistance on a global scale is complicated by the species’ remarkable genetic diversity. A comprehensive genomic analysis highlighted significant genetic diversity and distinct evolutionary lineages among global isolates.26PubMed. A comprehensive analysis of the epidemiological and genomic characteristics of global Serratia Marcescens This diversity means that resistance patterns can differ substantially between hospitals, between countries, and even between different wards in the same institution. A strain circulating in a neonatal unit may carry a completely different set of resistance genes than one causing trouble in an adult ICU down the hall.
For clinicians and microbiologists, this variability reinforces the importance of local antibiograms, hospital-specific summaries of what drugs work against the organisms they actually encounter. National and international surveillance data provide useful trend lines, but they can mask the heterogeneity that matters at the bedside. The message for patients and families is simpler: if you or someone you know is diagnosed with a Serratia infection, the specific resistance profile of that isolate determines treatment more than any general guideline can.