Hospital Superbug’s Iron Sensor Revealed as New Target Against Antibiotic Resistance

Pseudomonas aeruginosa has long been one of the most feared residents of modern hospitals. The bacterium thrives in the kinds of environments that intensive care units know all too well: burn wounds, ventilated lungs, catheters, and the compromised tissue of patients whose immune defenses are already stretched thin. It is a leading cause of hospital-acquired infections, and over the past decades it has grown steadily more resistant to the antibiotics clinicians rely on. Now, a team led by chemist Aaron Smith, professor of chemistry and biochemistry at the University of Maryland, Baltimore County, together with collaborators at Oklahoma State University, has uncovered a molecular mechanism that sits at the heart of how this pathogen manages one of its most essential resources: iron. The work, published in Nature Communications, describes a two-protein switch that senses iron both outside and inside the bacterial cell and, in response, dramatically rewrites the microbe’s metabolism and its defenses.

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