Scientists from SCELSE at Nanyang Technological University, Singapore (NTU Singapore) and Imperial College London have discovered how a hard-to-treat bacterium linked to hospital-acquired infections loads toxins into a molecular weapon used to attack other cells.

The finding lays the groundwork for researchers to design new ways to weaken the bacterium by disrupting this “weapon loading” mechanism, rather than trying to kill the resistant bacterium directly with antibiotics.

The bacterium, Pseudomonas aeruginosa is an opportunistic pathogen that can cause serious infections, particularly in people with weakened immune systems or chronic lung conditions such as cystic fibrosis. It is also known for its resistance to multiple antibiotics.

The findings, published in Nature Microbiology today, reveal how a syringe-like system in the bacterium, called the Type VI Secretion System or T6SS, loads toxin-carrying rings into a tube-like structure that can deliver multiple toxic proteins in one strike like a molecular speargun.

The T6SS allows P. aeruginosa to deliver different toxic proteins to attack rival microbes and immune cells of the body. This gives the bacterium a two-pronged advantage: it can damage and outcompete beneficial bacteria that normally live in a person, while also disrupting the body’s immune responses that would otherwise help clear the infection.

How the bacterial delivery system is loaded

The research project, jointly led by Prof Alain Ange Marie Filloux, Research Director at NTU’s Singapore Centre for Life Sciences Engineering (SCELSE) – an interdisciplinary biofilm and microbiome research centre, and Assoc Prof Tiago Dias da Costa from Imperial College London’s Department of Life Sciences at the Faculty of Natural Sciences, found that the T6SS is assembled through a step-by-step loading process.

Until now, it was unclear how specific toxic proteins were selected, routed and physically packed into the T6SS tube before being “injected” into immune cells or other competing bacteria. This study resolves a previously missing step in the assembly of this bacterial weapon.

Using biochemical and cryo-electron microscopy analysis, the team found that toxins are first captured by a protein called Hcp. Five additional Hcp proteins then wrap around the toxin to form a ring. Each toxin-loaded ring resembles a flying saucer, with the toxin positioned in the centre. In some cases, two rings may be needed to fully enclose a larger toxin.

Many rings can then stack on top of one another in a long tube. In P. aeruginosa, this structure can contain up to about 100 rings, allowing the bacterium to pack a high payload of toxins into a single delivery system.

When the system contracts like the plunger of a syringe being pushed, it forces out the stack of toxins in the tube, delivering the toxins into a target cell. As each tube can carry different toxins, a single firing event may deliver a cocktail of toxins with complementary or synergistic effects.

Prof Filloux, who is also from NTU’s School of Biological Sciences and Lee Kong Chian School of Medicine, said: “This bacterium does not just fire a single toxin. It loads a cocktail of toxins into a microscopic speargun and fires them in one strike, allowing it to attack different targets, including beneficial bacteria that normally live in the body, as well as the host’s own defence cells.

“This helps the bacterium colonise the host more effectively. If we can block this loading step in future, it could pave the way for approaches that disarm the bacterium and make it less able to cause disease.”

A possible route to disarming bacteria

The discovery points to a possible anti-virulence strategy against P. aeruginosa. Instead of killing bacteria directly, anti-virulence approaches aim to weaken the mechanisms that help them cause disease. This could reduce the selection pressure that drives antibiotic resistance, although further studies are needed to test whether the approach can work. The researchers said the next step is to study the signals and molecular interactions that control how the T6SS is loaded and fired.

Assoc Prof Costa said: “What is exciting about this work is that we can now see, at near-atomic detail, how a bacterial toxin is physically captured and enclosed inside the building blocks of the T6SS.

“High-resolution 3D images obtained from cryo-electron microscopy reveal that toxin loading is not a passive process, but a highly organised assembly pathway in which the secretion tube forms around its cargo. This gives us a molecular explanation for how P. aeruginosa prepares a cocktail of toxic effectors before firing them into competing microbes or host cells. By understanding this mechanism, we not only uncover a fundamental principle of bacterial cell biology but also identify new ways in which these systems might eventually be disrupted or repurposed.”

Beyond its possible future applications, the study also advances fundamental understanding of how bacterial cells route toxins from where they are made, across the cell envelope and into target cells.

The researchers said the mechanism could provide new molecular and cellular concepts that may one day be reflected in microbiology textbooks.

The findings could also support future research on T6SS-engineered “good” bacteria that protect healthy microbial communities, such as those in the gut.

One possible approach is to design harmless bacteria that can target specific harmful microbes while preserving beneficial ones, although such applications remain to be developed.

The research was supported by the UK Medical Research Council, the Wellcome Trust, the Singapore Ministry of Education, and Singapore’s National Research Foundation.

Paper titled “Molecular basis of type VI secretion system effector loading”, published in Nature Microbiology, on 27 May 2026. DOI: 10.1038/s41564-026-02363-x