RGCB Scientists Show How a Pneumonia Toxin Rides on Cell-Shed Vesicles to Damage Neighbouring Cells

When a cell is attacked by a bacterial toxin that punches holes in its membrane, one of its standard defences is to cut away the damaged patch and throw it out as a tiny bubble, or microvesicle. It is a repair job. New work from Thiruvananthapuram suggests that, for one of the most important toxins in pneumonia, this clean-up may be helping the damage spread.

Scientists at the BRIC-Rajiv Gandhi Centre for Biotechnology (RGCB) report in the Journal of Extracellular Vesicles that pneumolysin, a toxin made by Streptococcus pneumoniae, stays active on the vesicles that cells shed, and can damage the next cell those vesicles reach. The findings were publicised on 25 October 2026.

The toxin and the defence

S. pneumoniae — the pneumococcus — is a leading bacterial cause of pneumonia, as well as meningitis and bloodstream infections. Pneumolysin is one of its main weapons. It belongs to a family of pore-forming toxins that bind cholesterol in cell membranes, assemble into rings and open holes that can kill the cell.

Cells exposed to sub-lethal amounts of such toxins can survive by shedding the toxin-damaged membrane as microvesicles. The RGCB team asked what happens to the toxin after it leaves.

What the study found

The researchers report that:

• Pneumolysin is enriched in plasma membrane-derived microvesicles shed by cells under sub-lethal toxin exposure.
• Rather than being inert waste, the toxin remains attached to the vesicle membrane and keeps its biological activity.
• When these vesicles reach neighbouring cells, they can engage and damage the recipient membrane, carrying the toxin's effect beyond the cell that was first attacked — a route the paper describes as damage "beyond canonical pore formation".

To see how the transfer happens, the team worked with two other institutions. Cryo-electron microscopy carried out with Somnath Dutta and colleagues at the Indian Institute of Science, Bengaluru, showed pneumolysin positioned at the sites where vesicle and target membrane meet. Molecular simulations with researchers at TIFR Hyderabad suggested that an exposed region of vesicle-bound pneumolysin directly grips lipids in the recipient membrane, bending and thinning it and letting water penetrate. When the team altered key residues in that region, the membrane deformation fell substantially.

The paper lists Aswathy C. Sagilkumar as first author and Karthik Subramanian as senior author.

Building on earlier work

The study extends a line of research from the same group. A 2023 preprint, later published in iScience, reported that toxin-loaded vesicles shed by monocytes during challenge could fuse with recipient cells and cause cell death, and that CD4+ T cells were more susceptible than monocytes. The new paper adds the structural and molecular detail of how the vesicle-bound toxin engages a new membrane.

Why it matters

Most strategies against pneumolysin focus on the free toxin — antibodies that neutralise it, or vaccines that include toxin-derived antigens. If a meaningful share of the damage during infection is carried on host-derived vesicles, those approaches may need to account for a second, membrane-bound form of the toxin. The identification of specific residues that drive membrane engagement also points to a possible target for molecules that block the vesicle-to-cell step.

The work is laboratory and computational research; it does not yet show how large this pathway's contribution is in patients. But it is a good example of Indian institutions combining strengths — cell biology at RGCB, cryo-EM at IISc and simulation at TIFR — to answer a mechanistic question with global relevance. Pneumonia remains one of the biggest infectious killers of young children and older adults, and India carries a large share of that burden.

Sources

• ThePrint (PTI): RGCB scientists uncover hidden pathway for spread of pneumonia toxin between cells — https://theprint.in/india/rgcb-scientists-uncover-hidden-pathway-for-spread-of-pneumonia-toxin-between-cells/3081019/
• UNI: RGCB scientists uncover new pathway for spread of pneumonia toxin — https://www.uniindia.com/south-india/technology-kerala-pneumonia/224751
• Journal of Extracellular Vesicles (Wiley): Extracellular Vesicle-Bound Bacterial Toxin Pneumolysin Triggers Membrane Engagement and Damage Beyond Canonical Pore Formation — https://isevjournals.onlinelibrary.wiley.com/doi/10.1002/jev2.70387
• iScience: Bacterial pore-forming toxin pneumolysin drives pathogenicity through host extracellular vesicles released during infection — https://www.cell.com/iscience/fulltext/S2589-0042(24)01814-5
• bioRxiv preprint (2023) — https://www.biorxiv.org/content/10.1101/2023.10.12.561978v1