In their constant fight against antibiotic resistant bacteria, scientists have discovered a key secret of how Polymyxin B (PmB) is one of medicine last resort antibiotic killing demonstrating the mechanism of action. A joint effort by the researchers at Imperial College London, University College London, and University of Nottingham has identified that the bactericidal activity of Polymyxin B is bacterial energy dependent and was published in Nature Microbiology (2025). As opposed to popularly thought PmB does not merely puncture the membranes of the bacteria but rather uses the cellular metabolism against itself, killing it as a result.
The finding does not only transform our perception of the mechanism of action of antibiotics but also might help to develop more specific, energy sensitive drugs to combat antibiotic resistant diseases.
Knowing Polymyxin B: The Ancient Drug that Has a Secret.
Polymyxins Polymyxin B and Colistin (Polymyxin E) are antibiotics that have the ability to treat Gram negative bacteria, including E. coli and Pseudomonas aeruginosa. The drugs are used as a final resort in treating multidrug resistant pathogens. PmB acts by attacking lipopolysaccharides (LPS) complex fat sugar molecules which create a shell on the outside membranes of bacteria. PmB is bound to LPS which interferes with the bacterial envelope and kills the cell. But something puzzling to doctors has always been the fact that although PmB can be a highly effective treatment of bacteria in the laboratory, it does not always succeed when used within the human organism, particularly against dormant or so called persister cells, which are difficult to treat.
This mystery is solved by this new research.
Important Discovery: The Trigger is Bacterial Energy. The researchers established that the lethal effect of Polymyxin B is based on the metabolic activity of the bacterium the ability to generate energy (ATP).
Scientists compared cells in the two states in experiments with E. coli:
Exponential phase cells: active and metabolically active.
Stationary phase cells: hapless or fatigued.
Active cells perished quickly when it was exposed to Polymyxin B, but the dormant ones did not unless glucose, a simple sugar that increases metabolism, was provided. This showed that the killing process of PmB requires energy a breakthrough discovery that does not agree with the decades of presumptions of how the membrane targeting antibiotics act.
The Mechanism of how Polymyxin B Kills: Step by Step.
The experiment involved the most modern imaging methods such as the Atomic Force Microscopy (AFM) and fluorescent markers to observe a real time process of bacterial destruction.
And now is what the scientists saw:
Attachment and Extracellular Membrane Destruction.
The initial step is the adherence of PmB to the bacteria surface, which leads to minor disruptions without the killing of the cell. This contact causes large scale perturbations in energy rich bacteria, such as outer membrane protrusions and LPS loss of the protective coat.
Energy Dependent LPS Loss
LPS degradation necessitates synthesis and transportation with the aid of ATP. The inability to recycle the outer membrane in the cell makes PmB incapable of further penetration.
Gaining Access to the Inner Membrane (IM).
After LPS barrier collapses, PmB enters the inner membrane destroying it permanently. Curiously, this last step does not need any energy, it occurs spontaneously when PmB breaks through.
This process involves two steps the outer membrane disruption, which is energy dependent, and the inner membrane collapse, which is passive (that is, does not require energy).
The MCR-1 Resistance Gene and Role of LPS.
One of the key aspects of the study concerned the mechanism of Polymyxin B resistant bacteria survival. The team examined the MCR-1 gene which is a popular resistance factor that alters the LPS to block antibiotic attachment.
In the case of LPS loss did not happen when E. coli had the MCR-1 gene, and Polymyxin B was unable to attack the inner membrane despite surface changes still being observed. The result points to the strong defense of MCR-1: it interferes with the entry mechanism of PmB, which provides protection to the bacterium itself.
The Vulnerability of Energy Fuels: The Importance of this.
This fact alters the perception of antibiotic resistance. Most bacteria do not survive through mutation and instead enter in energy saving dormant phases. As PmB needs energy to cause its attack, these dormant microbes are able to wait the treatment this is the way to explain relapses and chronic infections. New therapies can reinstate the full potential of Polymyxin B by either attacking bacterial metabolism or causing dormant bacteria to wake up.
For instance:
Increased bacterial metabolism that is transiently induced by drugs might predispose infections to antibiotics.
Integration of PmB and the compounds that inhibit the production of LPS could enhance its lethal action.
A Sneak preview of Future Antibiotic Design.
The implication goes way beyond Polymyxin B. Most of the available drugs might be reconsidered on energy dependence. This would inform the design of the next generation of antibiotics that would act against bacteria at a time that is the most susceptible. Besides, the finding favors the use of an older drug, novobiocin, in order to predispose bacteria to polymyxins through the promotion of LPS transport and destabilization of the membrane. Concisely, it is not only scholarly to learn about bacterial energy but it is also the solution to recover the dead-beat antibiotics which are believed to be ineffective.
Conclusion: Reconsidering the Rules of Antibiotic Action.
The perspectives of the study by Nature Microbiology (2025) radically redefine the way Polymyxin B kills bacteria. Instead of being a direct membrane destroying device, PmB uses the metabolic activity of the bacterium to disarm it. This energy sensitive weakness has given scientists a window of opportunity to design smarter military measures against bacteria resistant to antibiotics without necessarily developing new drugs. With the growing global resistance to antibiotics in question, such results constitute a critical breakthrough in that matter a signal that in some cases, the answer to the question of how to kill bacteria is itself in the way they live.
Reference
Borrelli, C., Douglas, E. J. A., Riley, S. M. A. et al. Polymyxin B lethality requires energy dependent outer membrane disruption. Nat Microbiol (2025). https://doi.org/10.1038/s41564-025-02133-1