Scientists at Gladstone Institutes have discovered an unusual bacterial immune system that operates in almost the opposite way of previously known defenses. The surprising mechanism allows bacteria to activate a deadly self-destruct process when viruses attack, potentially offering new ideas for developing medical treatments and gene-editing technologies.
The findings, published in Molecular Cell, reveal how a group of bacterial defense systems called “type VI retrons” use an unexpectedly complex sequence of molecular events to fight infection. Unlike other retrons, these systems rely on DNA to activate a toxin rather than keep one suppressed.
Understanding this unusual process could do more than expand scientists’ knowledge of bacterial immunity. It may also provide a foundation for designing more effective DNA-editing tools, potentially allowing researchers to correct harmful genetic mutations with greater precision.
“This is, by far, the strangest of the retron systems that we or anyone else has come across,” says Gladstone Investigator Seth Shipman, PhD, senior author of the new study. “These retrons operate with Rube Goldberg-level complexity, using molecular mechanisms that we would never have thought to invent, but that we’ll use in future technologies.”
How Bacteria Sacrifice Themselves to Stop Viruses
Bacteria face constant threats from viruses known as phages, which infect bacterial cells and use their machinery to reproduce. Over time, bacteria have evolved a variety of defenses to prevent these viruses from spreading.
Among these defenses are retrons, molecular systems that can detect certain viral infections and trigger the infected bacterium to destroy itself. Although this sacrifice kills the individual cell, it prevents the virus from producing more copies and infecting nearby bacteria, helping protect the larger colony.
Retrons are also of considerable interest to scientists developing new genetic technologies. They can efficiently produce and deliver single-stranded DNA, making them promising tools for editing DNA inside human cells. Shipman’s team and other researchers are investigating whether these capabilities could eventually be used to correct mutations responsible for disease with exceptional accuracy.
Until now, scientists believed they understood the basic way retron defenses operate.
In previously studied retrons, a toxic protein is already present inside an uninfected cell, but its harmful effects are held in check by short pieces of single-stranded DNA. When a phage infects the bacterium, it disrupts this protective arrangement, allowing the toxin to act. The resulting cell death prevents the virus from spreading.
The newly investigated type VI retrons, however, did not appear to follow this familiar pattern.
A Bacterial Immune System That Works Backward
Researchers first became suspicious that type VI retrons were different when they could not detect the single-stranded DNA normally associated with these systems.
“The way we typically detect retrons inside uninfected cells is by looking for this single-stranded DNA,” says Karen Zhang, a PhD student in Shipman’s lab and lead author of the study. “But type VI retrons caught our attention because they didn’t appear to have this signature. It was very odd, and we wondered if they might be broken.”
Despite their unusual behavior, experiments showed that bacteria carrying type VI retrons could still defend themselves against phage infections. The systems were clearly functional, even though they lacked the expected molecular signs of activity.
“That kicked off a long molecular odyssey,” Shipman says. “Whenever we tried the experiments we normally use to study retrons, they would fail, so we kept having to think of new ways to figure out how these retrons work.”
After extensive investigation, the team discovered that type VI retrons reverse a key step in the usual defense process.
Rather than maintaining a substantial supply of single-stranded DNA and toxin before infection, these retrons produce little detectable DNA and no noticeable amount of toxin in healthy cells. The arrival of a phage triggers a sharp increase in DNA production.
As the DNA accumulates, it interacts with an RNA molecule that has been preventing the toxin from being made. This interaction removes the restriction, allowing the bacterium to rapidly produce the toxic protein.
The toxin appears to be an exceptionally small protein that inserts itself into the bacterium’s surrounding cell membrane. By disrupting the membrane’s stability, it causes the infected cell to die, stopping the viral infection from spreading.
In other words, the DNA acts as an activation switch rather than a restraint. Instead of preventing an existing toxin from causing harm, it helps turn on toxin production only when the bacterium detects a threat.
“It’s almost the mirror-image version of how we traditionally view retrons, because the DNA switches on toxin production instead of inhibiting an already-produced toxin,” Zhang says. “It would be fascinating to explore how bacterial evolution could have resulted in such a complex molecular system — whether type VI retrons are older or newer than other retrons, or whether they’re a stepping stone between different types.”
An Unusual Discovery Required New Experiments
Figuring out how type VI retrons function proved unusually difficult. Many established techniques that scientists use to examine retrons repeatedly produced no useful results, forcing the team to develop new experimental strategies and adapt existing methods.
One particularly important challenge involved detecting the DNA produced by type VI retrons in cells that had not been infected.
After conventional tests failed, the researchers tried a technique called Spacer-seq, which had previously been developed in Shipman’s laboratory. The method uses components of CRISPR-Cas, another bacterial immune system, to record molecular activity inside cells.
CRISPR-Cas is best known for its role in modern gene-editing technologies, but bacteria originally evolved these systems as defenses against viruses. Spacer-seq takes advantage of this natural machinery to help researchers investigate what happens inside cells.
The alternative approach finally revealed a trace amount of single-stranded DNA that earlier experiments had missed.
“We had just about given up hope that any DNA was being made when this approach finally revealed a very, very low level of production in uninfected cells,” Zhang says.
This breakthrough encouraged the researchers to continue investigating. Additional experiments confirmed that type VI retrons produce DNA at very low levels under normal conditions and substantially increase production only after a phage enters the cell.
The findings explained why the defense system had appeared inactive in earlier tests and revealed the unusual sequence of events responsible for its antiviral activity.
Could This Strange Bacterial Defense Lead to New Medical Treatments?
The discovery is already influencing research in Shipman’s laboratory, where scientists are exploring ways to transform the type VI retron’s natural machinery into useful technologies.
One promising application involves developing synthetic molecular sensors and switches that can regulate gene activity. Such systems could potentially allow researchers to control when particular genes are activated or suppressed, opening new possibilities for studying disease and designing treatments.
“This system represents a unique way to potentially build a molecular sensor and switch encoded in DNA and RNA, that no one has thought of before,” Zhang says. “Synthetic sensors and switches have many possible applications, both in treating disease and in designing experiments to better understand how certain diseases operate.”
Another possibility involves the retron’s unusually small but powerful toxins. Because these proteins can disrupt bacterial membranes and kill cells, researchers are investigating whether they could be adapted into new antimicrobial treatments.
Such therapies could potentially target harmful bacteria responsible for human infections, providing a new approach to combating bacterial disease.
These medical applications remain under development, but the discovery offers researchers a molecular design that would have been difficult to imagine without studying nature’s existing solutions.
“This study highlights why it’s so important for a molecular technology development lab like ours to keep looking to nature for inspiration,” Shipman says. “We can use what evolution has given us as starting points to build complex machines that would have been tremendously hard to even conceptualize.”
The research also raises intriguing questions about bacterial evolution, including how such an elaborate defense arose and whether type VI retrons represent an earlier, later, or intermediate stage in the evolution of these immune systems.
For scientists seeking new ways to manipulate DNA, control genes, and fight infections, this backward-acting bacterial defense could provide an unexpected starting point.
The research, “Reverse transcribed ssDNA derepresses translation of a retron antiviral protein,” was published in Molecular Cell on October 13, 2026.
Along with Shipman and Zhang, the authors include Matías Rojas-Montero, Darshini Poola, and Alejandro González-Delgado of Gladstone; Josepha Klas, Arturo Carabias, Dennis Zhang, Mario Mestre, Ruiliang Zhao, Ana Dávila-Hidalgo, Guillermo Montoya, and Rafael Pinilla-Redondo of the University of Copenhagen; and Lieza Chan and Kliment Verba of UC San Francisco.
The research received support from the National Science Foundation, the Robert J. Kleeberg Jr. and Helen C. Kleberg Foundation, the Gordon and Betty Moore Foundation, the Lundbeck Foundation, the Fulbright U.S. Student Program, the Junta de Castilla y León and European Social Fund Plus, the European Molecular Biology Organization, the Novo Nordisk Foundation, the European Research Council, and the Villum Foundation.
