Iron is not a luxury for living organisms; it is a necessity. Like most forms of life, Pseudomonas aeruginosa cannot survive without it, because iron serves as a critical cofactor in enzymes that drive respiration, DNA synthesis, and countless other core processes. But bacteria have a strong preference for a particular form of the element: reduced, ferrous iron, known chemically as Fe2+. This form of iron has beneficial chemical properties for microbial life, but it comes with a catch. Ferrous iron is sensitive to oxygen, which means it is only abundant in low-oxygen microenvironments. Those environments are far more common in the biological world than one might expect. Dental plaque, the lining of the gut, the thick coatings that form on the lungs of people with cystic fibrosis, and the tissue at the site of burn wounds are all places where oxygen is scarce and ferrous iron is plentiful. Not coincidentally, these are also places where bacteria congregate and where infections become difficult to treat.

Bacteria in these settings often build another layer of protection for themselves: biofilms. These are structured communities of microbes encased in a self-produced matrix, a kind of protective coating that shields the cells from both antibiotics and the immune system. Biofilms are a major reason why Pseudomonas infections are so stubborn and why the bacterium has become such a prominent symbol of the antibiotic resistance crisis. A drug that easily kills free-floating bacteria in a laboratory dish may fail completely against the same organism once it has settled into a biofilm on the lungs or on a medical device. Any strategy that could force bacteria out of these coatings, or prevent the coatings from forming in the first place, would represent a meaningful advance in the fight against resistant infections.

Because iron is so essential, one of the strategies that host organisms use to fight invading microbes is nutritional immunity: simply depriving them of the metal. The human body sequesters iron tightly, making it one of the scarcest nutrients available to a pathogen. As a result, bacteria have evolved elaborate and sensitive machinery to monitor how much iron is available to them and in what chemical form. The system studied by Smith’s team is one such mechanism, and the new paper reveals that it is far more sophisticated than previously appreciated. It consists of two proteins working in partnership. The first, called BqsS, sits in the bacterial cell membrane and detects iron outside the cell. When it senses ferrous iron in the environment, it passes a signal across the membrane to a second protein, BqsR, which resides inside the cell. Acting as a response regulator, BqsR then binds to DNA and switches genes on or off, steering a wide range of cellular functions in response to the iron signal.

This architecture, in which a membrane-bound sensor communicates with an intracellular DNA-binding regulator, is a classic example of a two-component signaling system, one of the dominant ways bacteria sense and respond to their surroundings. What surprised the researchers was not the existence of the system but the sheer breadth of its influence. The team confirmed that BqsR and its partner regulate iron uptake into the cell, as expected, but the analysis showed that the system does far more than manage import. It reaches into many corners of the bacterium’s genetic program, altering metabolism and behavior on a large scale. Smith described the scale of the rewiring with some astonishment, noting that the presence of this single ion changes all sorts of genes and that the system, in his words, has hands in a lot of different pies.

The second major surprise came from a closer look at BqsR itself. The researchers found that the regulatory protein can bind iron directly inside the cell. This gives the system an internal sensing capability on top of the external one provided by BqsS. When iron levels inside the bacterium rise too high, BqsR binds the iron and releases its grip on the DNA it had been attached to, switching off the genes it had been controlling. The result is a remarkably elegant feedback design: the system monitors iron outside the cell through its membrane sensor, monitors iron inside the cell through its regulator, and uses both streams of information to fine-tune gene expression. Smith likened the arrangement to a multi-layered cake, with all these different layers of sensing happening within one system. This kind of nested control allows the bacterium to respond not just to whether iron is present in its surroundings, but to the full dynamic of how iron levels fluctuate as it takes the metal up and metabolizes it.

The discovery carries obvious therapeutic implications. Because the same system that manages iron also controls biofilm formation, disrupting it could force bacteria out of their protective coatings and leave them exposed and vulnerable to antibiotics that are already in clinical use. Rather than designing an entirely new drug to kill the bacterium directly, researchers could aim to disarm its sensory machinery, stripping away the defenses that make it so hard to eradicate. Importantly, the reach of this finding may extend well beyond Pseudomonas. Related two-component systems exist in other harmful bacteria, including Vibrio cholerae, the organism that causes cholera. That means insights gained from studying the BqsS-BqsR pair could inform strategies against a broader family of dangerous pathogens, making this line of research broadly applicable across infectious disease.

Behind the scientific results is a story about how the research itself was done. The study was led by first author Alexander Paredes, who completed his Ph.D. at UMBC in 2025 and holds the distinction of being the first UMBC graduate student named an HHMI Gilliam Fellow, an early career initiative that supports Ph.D. students and their faculty advisors as they pursue ambitious science and build inclusive training environments. Paredes is now a postdoctoral researcher in the laboratory of chemist Squire Booker at the University of Pennsylvania. The project also drew on the labor of undergraduates and other graduate students in Smith’s lab, along with collaborators at Oklahoma State University and Reed College. Smith was emphatic about their contribution, saying the team could not have done the work without them, and describing publications of this kind as all-hands-on-deck efforts. In his view, research experience prepares students to be more critical thinkers and better problem solvers, an educational dividend that runs alongside the scientific one.

The team now has the resources to push the work further. A new grant from the National Institutes of Health will allow Smith’s group, again in collaboration with colleagues at Oklahoma State, to dig deeper into the molecular details of the iron-management system. The interdisciplinary team plans to examine how the membrane protein BqsS and its intracellular counterpart BqsR interact with each other, to identify which building blocks of each protein are most critical to their function, and to determine how the system responds to oxygen, a question made especially interesting by the oxygen sensitivity of the ferrous iron the system detects. Students in the lab will help test these ideas in living bacteria, moving the project from basic molecular understanding toward strategies that could one day improve the treatment of antibiotic-resistant infections. Reflecting on how the project came together, Smith credited both hard work and luck, and emphasized that solving problems which bridge biology, chemistry, and everything in between has been a deeply collaborative effort. For a pathogen that thrives precisely where chemistry and biology intersect, in the low-oxygen, iron-rich corners of the human body, it may be exactly that kind of interdisciplinary approach that finally turns its own sensory sophistication against it.

Subject of Research: Iron sensing by the BqsS/BqsR two-component system in Pseudomonas aeruginosa and its role in metabolism and biofilm formation

Article Title: UMBC-led team reveals how a common hospital pathogen senses iron—opening a new avenue for fighting antibiotic resistance

Article References: UMBC-led team reveals how a common hospital pathogen senses iron—opening a new avenue for fighting antibiotic resistance. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Pseudomonas aeruginosa, iron sensing, BqsR, BqsS, two-component system, biofilms, antibiotic resistance, hospital-acquired infections, ferrous iron, gene regulation, Nature Communications, UMBC

The target of this bacterial maneuver is calprotectin, one of the most abundant antimicrobial proteins in the human body. Composed of the calcium-binding subunits S100A8 and S100A9, calprotectin makes up roughly forty percent of the cytoplasmic protein in neutrophils and is dumped rapidly into infected tissues. Scientists have long known that it sequesters manganese and zinc, but more recent work showed that it also binds ferrous iron, Fe(II), with high affinity. What remained unclear was whether this iron-withholding function actually matters during infection, and whether pathogens have evolved dedicated countermeasures. The new study answers both questions with unusually complete evidence, spanning structural biology, biophysics, genetics and mouse models.

The story began with a simple observation about stress. When the researchers deleted clpV1, a gene encoding the ATPase motor essential for the operation of Y. pseudotuberculosis’s type VI secretion system 1, or T6SS1, the mutant bacteria became markedly more sensitive to hydrogen peroxide and to acidic conditions at pH 5.0. Complementing the mutation restored normal resistance, and growth curves under non-stress conditions showed no baseline fitness defect, indicating that the secretion system itself was doing something specific to help the bacteria weather oxidative and acidic assault. Both forms of stress are highly relevant inside a host, where neutrophils bombard invaders with reactive oxygen species and the stomach presents a formidable acid barrier.

To find out what T6SS1 was secreting, the team systematically scanned the gene cluster for small hypothetical proteins, prioritizing uncharacterized open reading frames at the cluster terminus, a region known to harbor lineage-specific accessory genes. That search yielded YPK_0411, a diminutive 93-residue protein that structural modeling suggested might resemble iron-sulfur cluster-binding proteins. Secretion assays using a VSVG tag detected the protein in culture supernatants of wild-type bacteria but not of the clpV1 mutant, confirming that its export depends on the secretion machinery. The researchers named the protein SfeP, for T6SS-secreted ferrous iron-binding effector protein.

The biophysical characterization of SfeP is where the study becomes technically compelling. Using isothermal titration calorimetry under carefully controlled, oxidation-minimizing conditions, with ferrous iron freshly prepared in deoxygenated buffer supplemented with ascorbate and verified by ferrozine assays, the team showed that SfeP binds Fe(II) with a dissociation constant of about 1.013 nanomolar, an extraordinarily tight interaction. Just as telling was the specificity: SfeP ignored magnesium, manganese, zinc, copper and nickel, and it preferred Fe(II) over Fe(III). Inductively coupled plasma mass spectrometry then showed that deleting sfeP specifically depleted intracellular iron while leaving magnesium, zinc and manganese pools untouched, exactly the signature expected of a dedicated iron acquisition factor.

Functionally, the sfeP mutant grew normally in rich medium but faltered when iron was chelated away with EDDHA and EDDA, and the defect could be rescued either by restoring the gene or by flooding the medium with ferrous iron. The mutant was also hypersensitive to peroxide and acid, and, crucially, supplemental Fe(II) protected wild-type and complemented strains under those stresses but offered the mutant far less benefit, consistent with a broken uptake pipeline. A clever solvent-partitioning assay using chloroform affinity suggested that the mutant carried a reduced intracellular pool of electron-donating species, again pointing to diminished ferrous iron. Together these experiments establish that SfeP is not merely an iron-binding curiosity but a working component of an iron import pathway.

To understand how SfeP actually moves iron across the bacterial outer membrane, the team solved its crystal structure at 1.76 angstrom resolution. The protein folds into a compact four-helix bundle, an architecture reminiscent of copper storage proteins such as Csp3 and Csp1, but without their cysteine-rich metal-binding motifs, and without the ferroxidase centers of ferritins or the dimeric iron-coordination interfaces of encapsulin cargo proteins. No bound iron was visible in the electron density, likely because ferrous iron oxidizes readily and associates transiently, so the structure serves as a framework while the calorimetry establishes the binding chemistry. The authors lay out a careful roadmap for future anaerobic co-crystallization and metal-substitution experiments to pin down the binding site itself.

The delivery partner turned out to be OmpF, a well-known outer membrane porin. A GST pull-down screen with SfeP-coated beads specifically retained a 40-kilodalton protein from bacterial lysates that mass spectrometry identified as OmpF, which shares 57 percent sequence identity with the ferrous uptake porin of Escherichia coli. Deleting ompF lowered both total and ferrous intracellular iron, and the interaction was confirmed by in vivo and in vitro pull-downs and far-western blotting with purified proteins. Molecular docking suggests SfeP binds near the pore region of OmpF, supporting a model in which the secreted effector captures extracellular Fe(II) and escorts protein-complexed iron through the porin into the periplasm. The authors propose the term ferrousophore for this class of secreted ferrous iron-binding protein, drawing an analogy to the HasA hemophore system that delivers heme to its outer membrane receptor.

The in vivo consequences are dramatic. When mice were infected orally with the sfeP mutant, every animal survived to 21 days, whereas fewer than ten percent of mice infected with wild-type bacteria lived that long. Histopathology revealed severe intestinal damage in wild-type infections, including mucosal abscission and epithelial disruption, that was absent in mutant infections, and the mutant could barely be recovered from the stomach, small intestine, cecum, spleen or feces. Even neutralizing stomach acid with bicarbonate only partially rescued the mutant, showing that SfeP contributes to virulence well beyond acid tolerance, supporting gut colonization and systemic spread. The mutant also formed weaker biofilms on living nematodes and abiotic surfaces, produced less extracellular polysaccharide, showed altered surface hydrophobicity and swam more actively, painting a picture of a factor that reshapes multiple lifestyle traits through iron homeostasis.

Perhaps the most elegant experiment addresses calprotectin directly. In calprotectin-deficient S100a9 knockout mice, the severe colonization defect of the sfeP mutant was largely reversed, with high bacterial burdens recovered from intestinal tissues, demonstrating that calprotectin-mediated iron withholding is a genuine force shaping infection outcomes. Conversely, the knockout mice proved more susceptible to wild-type Yersinia, confirming the protective role of the protein for the host. Because the rescue was incomplete, SfeP evidently also promotes fitness through calprotectin-independent mechanisms. The broader significance is twofold: it validates ferrous iron sequestration as a real pillar of nutritional immunity, and it expands the known repertoire of the type VI secretion system, already implicated in manganese, zinc and copper scavenging, into the ferrous realm. Given that SfeP homologs are distributed across many bacterial taxa and that OmpF is widespread, the ferrousophore strategy may be far more common than anyone suspected, and disrupting the calprotectin-iron axis or the SfeP-OmpF partnership could offer a new anti-virulence target that avoids the selective pressures of conventional antibiotics.

Subject of Research: T6SS-mediated ferrous iron acquisition by Yersinia pseudotuberculosis to evade calprotectin-mediated nutritional immunity

Article Title: Yersinia pseudotuberculosis secretes an Fe (II)-binding effector to evade calprotectin-mediated nutritional immunity

Article References: Dai, Q., Guan, H., Huang, J., Hou, J., Zhang, M., Wang, Y., Zhang, P., Xu, L., Gu, H., Wang, Y., Ouyang, S., & Shen, X. (2026). Yersinia pseudotuberculosis secretes an Fe (II)-binding effector to evade calprotectin-mediated nutritional immunity. Stress Biology, 6(1), Article 29. https://doi.org/10.1007/s44154-026-00304-6

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00304-6

Keywords: Yersinia pseudotuberculosis, type VI secretion system, SfeP, ferrous iron, calprotectin, nutritional immunity, OmpF, ferrousophore, oxidative stress, acid stress, virulence, iron homeostasis