According to the World Health Organization (WHO), antibiotic resistance in pathogenic bacteria is one of the greatest threats to human health. In light of this challenge, the development of new therapies has become crucial. In Strasbourg, the team co-led by Isabelle Schalk (DRCE, CNRS) and Gaëtan Mislin (DR1, CNRS) is pursuing an innovative “Trojan horse” strategy: introducing bactericidal molecules into bacteria by exploiting their own iron import system, as iron is essential for their survival.
Responsible for more than half a million deaths worldwide each year, Pseudomonas aeruginosa ranks among the five deadliest bacterial pathogens. Its resistance to many antibiotics poses a major challenge, particularly in hospital settings, where it primarily targets immunocompromised patients, patients with severe burns, those in intensive care, or those with invasive medical devices (catheters, tubes, ventilators). In people with cystic fibrosis, it causes chronic, often fatal, lung infections due to its ability to form persistent biofilms and resist treatment.
This bacterium, which can grow in water, soil, or moist surfaces, adapts to a wide variety of environments, including living organisms. To survive, it has developed—through natural selection—multiple adaptive mechanisms, including various pathways for acquiring iron, a nutrient essential to all living organisms.
Isabelle Schalk, a researcher at the Biotechnology and Cell Signaling (BSC) laboratory in Strasbourg, has been studying P. aeruginosa for more than 25 years. A specialist in iron uptake mechanisms, she explains: “Iron is essential to all forms of life, from bacteria to humans, because it plays a role in numerous biological processes. Unlike sugars or fats, which can be replaced by other molecules, iron has no equivalent: it is irreplaceable. And since it is scarce for cells, bacteria must constantly fight to obtain it. From the very beginning of my research into these mechanisms, I saw iron uptake as a promising therapeutic target: blocking access to iron could indeed weaken the bacterium. P. aeruginosa has, in fact, developed an exceptional ability to take up this element under a variety of conditions: it has 21 iron acquisition pathways—a number rarely seen in microorganisms—which makes the task particularly complex.”
To capture iron, bacteria produce siderophores—molecules that they release into their environment to capture iron and take it back into the cell. P. aeruginosa synthesizes two of these, but can also utilize more than twenty siderophores produced by other microbes through a hijacking strategy.
In the fight against antibiotic resistance, an innovative approach—known as the “Trojan horse” strategy—involves coupling antibiotics with these siderophores. The antibiotic is then transported into the bacterium along with the iron, thereby circumventing a major obstacle: the low permeability of the bacterial cell wall, which normally blocks the entry of molecules. This method is inspired by a natural mechanism: certain bacteria already produce hybrid molecules (siderophores + antibiotics) to eliminate their competitors.
Cefiderocol, an antibiotic used in hospitals and developed by Shionogi, chelates iron and uses iron uptake pathways to penetrate bacteria. Despite this initial success, many challenges remain in developing other effective siderophore-antibiotic conjugates.
In fact, P. aeruginosa does not simultaneously express all 21 of its iron import pathways; rather, it adapts and activates only those that will be most effective depending on the bacterium’s environment or the type of infection. To use these pathways as entry points for antibiotics, it is therefore essential to understand which pathway is expressed and used depending on the context or type of infection, and which siderophore to target for optimal delivery.
As Isabelle Schalk explains: “If we ever want to develop new antibiotics delivered by siderophores, we need to identify which pathways are activated by the bacteria, both at the population level and at the single-cell level. This raises a question: in a given environment, do all bacteria use the same iron import pathway, or is there heterogeneity within the population?”
To answer these questions, funding from the French National Research Agency (ANR) enabled the team to develop 21 strains of P. aeruginosa capable of emitting a fluorescent signal when one of the 21 iron import pathways is activated. This allows for real-time monitoring of the expression of these pathways in bacterial cultures. The Jean-Marie Lehn Foundation subsequently supported the project to develop a microfluidic system to study these mechanisms at the level of individual cells.
“This system allows us to study P. aeruginosa in a precisely controlled environment (iron concentration, pH, etc.) and to track the activation of the various iron uptake pathways,” explains the researcher, who has recruited a doctoral student and assigned an engineer to this project. This duo, working at the intersection of imaging and microfluidic chip design, will make it possible to predict the environmental conditions that specifically activate the multiple iron uptake mechanisms of P. aeruginosa.
In collaboration with the team led by Professor Morgan Madec (ICube Laboratory, Strasbourg), a specialist in the modeling of biological mechanisms, Isabelle Schalk aims to use artificial intelligence to predict which iron uptake pathways the bacterium activates depending on its environment. This approach will help identify the most promising pathways for the targeted delivery of antibiotics.
By combining fluorescent reporters and mathematical modeling, researchers on the team recently quantified the transcriptional regulation of four iron-specific transporters using catechol-type siderophores (article published in PNAS; see source below). The results reveal distinct induction mechanisms:
This research demonstrates how P. aeruginosa interprets environmental signals (concentration of catechol-type siderophores, iron availability) to finely tune its iron acquisition strategy.
These findings provide a better understanding of why cefiderocol, an antibiotic already in clinical use, does not always work optimally. Until now, doctors did not always understand why its effectiveness varied from case to case: our findings reveal that it depends, among other factors, heavily on environmental conditions, particularly the concentration of iron and monocatechols.
These findings are crucial: to use iron import pathways as entry points for antibiotics, it is essential to know which pathway is expressed and at what concentration of siderophore it responds. By defining these conditions, the researchers are paving the way for more targeted and effective therapies against multidrug-resistant bacteria.
This project once again illustrates the importance of funding basic research, which is the only way to ensure the development of innovative therapies in the long term. This commitment is shared by the Jean-Marie Lehn Foundation, which funds ambitious, innovation-driven exploratory projects every year.
Source:
Volck, F., Revillot-Schmidt, A.-E., Hubert, T., Crespin, V., Mislin, G. L. A., Cunrath, O., Madec, M., & Schalk, I. J. (2026). Distinct transcriptional mechanisms of catechol siderophore transporters in Pseudomonas aeruginosa for efficient iron acquisition. Proceedings of the National Academy of Sciences, 123(33), e2602150123. https://doi.org/10.1073/pnas.2602150123
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