Morphological and Behavioral Traits of Serratia marcescens

Serratia marcescens is a rod-shaped bacterium best known for producing a vivid red pigment called prodigiosin, but its repertoire of traits extends far beyond color. Found in soil, water, on plant surfaces, and in hospital settings, this organism can radically reshape its own body plan depending on its environment, wage chemical warfare against neighboring bacteria, degrade one of the toughest biological polymers on Earth, and cause disease in hosts ranging from humans to corals. Few single bacterial species display such a broad toolkit of morphological flexibility and behavioral complexity.

Two Bodies, One Bacterium

One of the most striking features of S. marcescens is its ability to switch between two fundamentally different cell types. When growing in liquid, the bacterium exists as a short rod carrying just one or two flagella, propelling itself through fluid in what microbiologists call swimming motility. But place those same cells onto a solid surface, and a transformation begins. The cells stop dividing normally, elongate dramatically, and sprout anywhere from ten to a hundred flagella along their sides.¹ These elongated, heavily flagellated “swarmer” cells are specialized for moving across surfaces in coordinated groups rather than swimming solo through liquid.

The switch is not just cosmetic. Swarmer cells behave collectively, moving outward in waves across agar plates in a way that individual swimmers never could. This dual lifestyle lets S. marcescens exploit both aquatic and surface environments, and it partly explains why the organism turns up in so many different habitats.

The Red Pigment and Its Temperature Switch

Prodigiosin, the pigment responsible for S. marcescens colonies’ characteristic red color, has fascinated observers for centuries. Early reports of “bleeding bread” and red-stained communion wafers were almost certainly caused by S. marcescens growing on starchy surfaces. But pigment production is not automatic. It is tightly controlled by temperature. Peak prodigiosin output occurs around 27°C, and no pigment forms when cultures are grown at 37°C or above.² This is why clinical isolates from human infections, which grow at body temperature, are often non-pigmented and were historically misidentified as other species.

The control mechanism operates at the level of gene transcription. At 37°C, a transcription factor called HexS suppresses the genes encoding the prodigiosin production pathway. When researchers forced those genes to turn on at 37°C using artificial promoters, the cells made pigment just fine, proving that the cellular machinery for assembling prodigiosin works at higher temperatures. The bottleneck is purely in gene regulation, not in the chemistry itself.³ Interestingly, if cells are held at 38°C for too long, they lose the ability to bounce back to pigment production even after shifting to a permissive temperature, suggesting that extended heat exposure causes irreversible changes in the population.²

What Prodigiosin Does Beyond Coloring Colonies

Prodigiosin is not just decorative. The pigment carries antibacterial, anticancer, and antioxidant properties that have attracted serious research interest. Against bacteria, prodigiosin is active against both gram-positive and gram-negative species, with particularly strong inhibition of Enterococcus faecalis at low concentrations.⁴ In cancer cell studies, purified prodigiosin has shown cytotoxic effects against liver cancer and breast cancer cell lines, though the doses involved are still far from anything used in a clinical setting.⁵ Broader reviews describe prodigiosin as having both anticancer and immunomodulatory activities, placing it among the more pharmacologically interesting bacterial secondary metabolites.⁶

For the bacterium itself, prodigiosin likely offers a competitive edge against other microbes sharing the same soil or water niche. Its antibacterial effects might help clear space for S. marcescens to grow, though this ecological role is still being investigated.

Surfactants, Swarming, and Surface Conquest

Swarmer cells alone cannot conquer a surface. S. marcescens also secretes a biosurfactant called serrawettin W1, an exolipid that reduces the friction between the bacterial colony and whatever it is growing on. The gene responsible, swrW, encodes a nonribosomal peptide synthetase, and mutants that lack serrawettin production lose the ability to swarm effectively.⁷ The biosurfactant essentially lubricates the path, letting the elongated swarmer cells glide outward in expanding rafts.

Researchers have even manipulated this swarming behavior using alternating-current electric fields. When the frequency matched the natural oscillation frequency of S. marcescens cells, swarming accelerated significantly, an effect not seen in bacteria that lack flagella or do not produce surfactant.⁸ This finding may eventually inform strategies for disrupting biofilm formation by controlling how cells attach to surfaces.

Navigating Chemical Gradients

S. marcescens does not just move randomly. It actively senses chemical signals in its surroundings and adjusts its swimming direction accordingly. When exposed to gradients of the amino acid L-aspartate, individual cells show a measurable bias in their tumble rate depending on whether they are swimming toward or away from higher concentrations. Researchers tracked large numbers of individual cells and confirmed that S. marcescens uses a relative gradient-sensing mechanism, meaning the cell compares the signal strength over time rather than detecting an absolute concentration.⁹

This chemotactic behavior has even been exploited in bioengineering. S. marcescens cells attached to microswimmers show chemotactic responses to L-serine that peak at a similar gradient concentration as those of E. coli, suggesting that the underlying signaling pathway is broadly conserved across these species.¹⁰ At higher concentrations, the chemotactic response drops off because the cell’s receptors become saturated.

Quorum Sensing and Coordinated Behavior

Many of S. marcescens’ most interesting behaviors are not carried out by individual cells but are coordinated across populations through quorum sensing, a chemical communication system based on small signaling molecules called N-acyl homoserine lactones, or AHLs. Cells constantly release these molecules, and as the population grows, AHL concentration rises. Once it crosses a threshold, genes controlling pigment production, nuclease secretion, biosurfactant output, and biofilm architecture all switch on or off in response.

The quorum sensing circuit in S. marcescens has an unusual twist. In most studied bacteria, the receptor protein that binds these signaling molecules acts as an activator, turning genes on when population density is high. In S. marcescens, the receptor homologue SpnR instead acts as a repressor, keeping target genes silent until the right AHL accumulates and lifts the repression.¹¹ Long-chain AHLs can also antagonize biosurfactant-mediated surface movement, adding another layer of regulatory nuance. Genome-wide searches across 25 complete S. marcescens genomes have identified three distinct classes of the signal-producing enzyme, suggesting that quorum sensing circuits vary among strains.¹²

Quorum sensing also governs how S. marcescens interacts with surfaces. On non-living materials, AHL-based signaling regulates adhesion and biofilm maturation. But on living human corneal epithelial cells, adhesion was not quorum sensing-dependent, pointing to distinct mechanisms for colonizing biological versus inert surfaces.¹³

Building Biofilms

The biofilm lifecycle of S. marcescens follows a distinctive pattern. Cells first form small microcolonies, which mature and eventually develop hollow interiors. Then, within less than a day, the biofilm undergoes a sudden expansion, radiating outward with a burst of new structural growth around days three to four. This rapid expansion generates phenotypic variants, some “sticky” and some not, driven by single-nucleotide changes in a gene involved in making the sugary polymers that hold the biofilm together.¹⁴ That genetic diversification within a single biofilm helps the population hedge its bets, producing subpopulations better suited to different conditions.

Biofilms also protect S. marcescens from predators. Microcolony-type biofilms resist grazing by certain flagellated protozoans, while filamentous chains that form under quorum sensing control protect against surface-feeding amoebae.¹⁵ Different biofilm architectures thus serve as defense strategies against different types of predators.

Breaking Down Chitin

S. marcescens is one of the most effective chitin degraders known in the bacterial world. Chitin, the second most abundant natural polymer after cellulose, forms the exoskeletons of insects and crustaceans and the cell walls of fungi. The bacterium’s chitinolytic machinery includes at least four chitin-active enzymes working in concert. Two processive chitinases, ChiA and ChiB, move along chitin chains in opposite directions like molecular sawblades, while a third enzyme, ChiC, makes random internal cuts. A fourth component, a lytic polysaccharide monooxygenase called CBP21, uses an oxidative mechanism to break open new chain ends on the crystalline chitin surface, giving the other enzymes fresh starting points. A final enzyme converts the resulting fragments into single sugar units.¹⁶

Combining ChiA and ChiB produces a synergistic effect, degrading chitin faster together than either enzyme alone, which makes sense given their complementary directionality.¹⁷ This enzyme system has become a model for studying how bacteria break down tough, insoluble polymers, and it has practical relevance for bioconversion of shellfish waste.

Killing Competitors with Molecular Spears

S. marcescens carries a type VI secretion system, essentially a spring-loaded molecular syringe that punctures neighboring bacterial cells and injects toxic proteins. This system is used aggressively against both other species and other strains of S. marcescens itself. In mixed cultures with the opportunistic pathogen Enterobacter cloacae, cells lacking a functional type VI secretion system were outcompeted, demonstrating that this weapon is essential for persistence in microbial communities.¹⁸

The toxins delivered through this system are diverse. One effector protein, Ssp6, forms cation-selective pores in the inner membrane of target cells, disrupting their electrical charge and increasing outer membrane permeability, which is effectively lethal.¹⁹ Other effectors belong to the Rhs family and include DNase toxins that shred the target cell’s DNA. These Rhs proteins turn out to be the primary determinant of whether one S. marcescens strain can outcompete another, making intraspecies warfare just as fierce as competition between different species.²⁰

A Pathogen That Crosses Kingdoms

S. marcescens is an opportunistic human pathogen, particularly in hospital settings, where it causes urinary tract infections, wound infections, respiratory infections, and bloodstream infections. Clinical isolates from different body sites show distinct virulence profiles. Strains from wounds and respiratory specimens tend to produce the most protease activity and form robust biofilms, while strains from urinary tract infections show stronger hemolytic activity.²¹

What makes S. marcescens unusual among bacteria is the breadth of hosts it can infect. A strain called PDR60, traced to human wastewater in the Florida Keys, was identified as the cause of acroporid serratiosis, a disease devastating to the endangered Caribbean elkhorn coral. Researchers fulfilled Koch’s postulates by showing that the wastewater-derived strain produced disease signs in coral fragments within four to five days, establishing what they described as the first documented marine “reverse zoonosis,” a human pathogen jumping to a marine invertebrate.²² Corallivorous snails and non-host corals may serve as reservoirs for the pathogen between outbreaks.

S. marcescens also attacks plants. It has been confirmed as a causative agent of cucurbit yellow vine disease in squash, producing characteristic symptoms after inoculation of healthy plants.²³ And in entomology, S. marcescens is a well-known insect pathogen whose virulence evolves in response to selection pressures both inside and outside its hosts.²⁴ This cross-kingdom versatility reflects the organism’s large and flexible genome rather than specialization for any one host type.

Resistance to Antibiotics and Disinfectants

S. marcescens is intrinsically resistant to several antibiotic classes, and it readily acquires additional resistance genes. Its chromosomal AmpC cephalosporinase provides built-in resistance to many older beta-lactam antibiotics, and clinical strains frequently carry extended-spectrum beta-lactamases and carbapenemases that defeat newer drugs.²⁵ A genomic study of carbapenem-resistant S. marcescens from a South African hospital found that most isolates carried OXA-48-like carbapenemases, with many simultaneously harboring multiple beta-lactamase genes.²⁶

Equally concerning is S. marcescens’ ability to resist hospital disinfectants. During a 2022 outbreak in a Hungarian intensive care unit, the outbreak strain survived standard cleaning with quaternary ammonium compound disinfectants. Genomic analysis revealed a constellation of genes encoding efflux pumps, biofilm production pathways, and stress response systems that together enabled environmental persistence despite regular disinfection.²⁷ Separate work confirmed that specific efflux pump genes confer resistance to quaternary ammonium compounds, reducing their killing power by roughly a hundredfold compared to susceptible strains.²⁸ This dual resistance to drugs and disinfectants is a major reason S. marcescens persists in hospital environments.

A Cold War History

The assumption that S. marcescens was harmless had real consequences. Because it was brightly pigmented and considered non-pathogenic, the U.S. military used it as a tracer organism in biological warfare simulation tests during the 1950s and 1960s, releasing it over populated areas including San Francisco Bay. When this program came to light in the 1970s, it triggered congressional hearings and contributed to a broader reckoning about military experimentation on civilians.²⁹ The organism’s reputation as a harmless saprophyte has long since been overturned.

Environmental Cleanup Potential

Not all of S. marcescens’ traits are problematic. Its metabolic flexibility makes certain strains useful for bioremediation of heavy metal-contaminated environments. One strain, HB-4, can adsorb over 90 milligrams of cadmium per gram of dry cells, using a combination of surface binding, accumulation in the space between its two cell membranes, and uptake into the cell interior. Even naturally dead cells retain most of this adsorption capacity, meaning biomass could potentially be used as a passive filter.³⁰ When paired with the plant Houttuynia cordata, the bacterium boosted the plant’s cadmium uptake by over 40% compared to plants growing alone.

Other strains use a different strategy entirely: they produce urease, which drives the precipitation of calcium carbonate, trapping dissolved cadmium within the mineral crystals. This approach removed over 95% of soluble cadmium from test media.³¹ Additional studies have confirmed that S. marcescens can biosorb lead, cadmium, and chromium, with lead removal being particularly effective.³²

A Remarkably Flexible Genome

Many of these diverse traits trace back to S. marcescens’ genomic architecture. Comparative analyses across strains reveal a core genome of roughly 3,000 genes shared across the genus Serratia, but with remarkable intraspecies genetic diversity layered on top.³³ The pan-genome of S. marcescens, meaning the total collection of genes found across all known strains, contains roughly 57,700 genes, the vast majority of which appear in fewer than 15% of strains.³⁴ This “open” pan-genome indicates constant gene gain through horizontal gene transfer, consistent with an organism that inhabits wildly different environments.

The genome also harbors hypervariable loci where large blocks of genes are swapped in and out, analogous to plasticity zones described in other pathogens. Gene flow does not stay within species boundaries either. Genes that are core to certain S. marcescens lineages have been found in members of other Serratia species, suggesting ongoing genetic exchange across species lines.³⁵ This genome-level flexibility is, in a sense, the ultimate behavioral trait: it is what allows S. marcescens to continually acquire and refine the diverse capabilities described throughout this article, from new resistance genes to novel toxins, keeping one step ahead of whatever niche it lands in.