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On Thursday, 38 prominent biologists issued a dire warning: Within a few decades, scientists will be able create a microbe that could cause an unstoppable pandemic, devastating crop losses or the collapse of entire ecosystems.

The scientists called for a ban on research that could lead to synthesis of such an organism.

“The consequences could be globally disastrous,” said Jack W. Szostak, a Nobel-prize-winning chemist at the University of Chicago who helped write a 299-page technical report on the risks of the research.

In an accompanying commentary in the journal Science, Dr. Szostak and his colleagues warned that an organism created with the new technology could cause “extraordinarily damaging consequences for the environment, agriculture, and human well-being.”

To make such a microbe, scientists would have to build a cell that defied one of the fundamental properties of life on Earth. The molecules that serve as the building blocks of DNA and proteins typically exist in one of two mirror-image forms. But living cells rely on just one form.

Our DNA, for example, has a backbone made partly of sugar. While sugar molecules can exist in left- and right-handed forms, DNA only uses the right-handed molecules.

That’s the reason DNA’s double helix has a right-handed twist. Our proteins, by contrast, are made of left-handed amino acids. This combination is found not just in humans, but in every species on Earth.

Scientists are still debating how evolution landed on this arrangement. In theory, a mirror cell — with left-handed DNA and right-handed proteins — could carry out all the biochemical reactions required to stay alive.

But as far as biologists can tell, mirror cells do not exist. At least, not yet.

In recent decades, chemists have discovered how to make mirror proteins. Researchers have welded together right-handed amino acids to create mirror versions of natural proteins made by our own bodies.

Mirror proteins behave much like their natural counterparts, with one important difference: They take much longer to break down. That’s because the natural enzymes that normally degrade proteins have shapes that are adapted for attacking left-handed proteins.

They cannot grip mirror proteins and cut them into fragments. Their failure is akin to what happens if you try to twist open a lid from a jar by turning it counterclockwise, only to discover that the threads on the jar twist in the opposite direction.

Chemists are now trying to exploit mirror proteins, hoping they can be used to create long-acting drugs for diseases ranging from H.I.V. to Alzheimer’s.

In recent years, scientists have taken even bigger strides forward in mirror biology. Ordinary cells make proteins by reading a gene, making a copy of the gene’s sequence in an RNA molecule, and shipping that RNA to a protein-making factory.

In 2022, Yuan Xu and Ting Zhu, two researchers at Westlake University in China, created mirror enzymes that can produce mirror RNA molecules by reading mirror genes. Similar advances have raised the prospect that scientists could eventually make all the parts required to build a mirror cell, perhaps in ten to thirty years.

“The creation of mirror-image life is one of the ultimate applications of synthetic mirror-image proteins,” Richard Payne, a chemist at the University of Sydney in Australia and his colleagues wrote last year.

Several teams of scientists started taking further steps toward mirror cells.

“It’s inherently incredibly cool,” said Kate Adamala, a synthetic biologist at the University of Minnesota. “If we made a mirror cell, we would have made a second tree of life.”

Aside from being cool, a mirror cell might also be medically valuable. Scientists could program it to make bigger, more powerful mirror proteins.

Kevin Esvelt, a biologist at MIT who studies the risks of biotechnology, had vaguely wondered in the past if mirror cells might pose a risk. As its synthesis became possible, he began to take that risk seriously.

He raised his concerns with biosecurity experts at Open Philanthropy, which funds research on potential threats to humanity such as pandemics and artificial intelligence.

They brought together Dr. Adamala and other researchers working on mirror cells, along with immunologists, plant biologists and evolutionary biologists, to talk about the possible risks.

The discussion felt at first like science fiction to Jonathan Jones, a plant biologist at the Sainsbury Laboratory in Norwich, England. “It took me a while to take it seriously,” he said.

But he eventually recognized the potential for a planet-wide catastrophe if a mirror cell escaped containment — either accidentally released from a lab, or set free as a biological weapon.

The researchers then spent weeks plowing through the scientific literature to see if they could falsify their hypothesis.

“We’ve all done our best to shoot it down,” said Vaughn Cooper, an evolutionary biologist at the University of Pittsburgh. “And we failed.”

The trouble with mirror cells is that they could probably evade most of the barriers that keep ordinary organisms in check. To fight off pathogens, for example, our bodies must first detect them with molecular sensors.

Those sensors can only latch onto left-handed proteins or right-handed DNA and RNA. A mirror cell that infected lab workers might spread through their bodies without triggering any resistance from their immune systems.

There wouldn’t be many organic molecules inside a human body for a mirror cell to feed on. But Dr. Cooper and his colleagues suspect that it might find enough to grow slowly. And if the immune system did not detect the growing infection, it could spread without limit.

“Ultimately, that host will be overrun, and that will be fatal,” Dr. Cooper said.

A victim of mirror cells would harbor a vast supply of the microbes, which could spread to other people and start a pandemic. And it would be one that medicine would be unlikely to stop.

An antibiotic typically works against ordinary microbes by locking onto their proteins or its DNA. Such a drug would probably be useless against a mirror cell, because the drug could not get a proper grip on an essential molecule.

Drug developers might be able to create mirror antibiotics, but the treatments might not be ready to use until a mirror pandemic was out of control.

Plants have their own pathogen detectors, which would also fail. “Essentially, all plants in the world would be unable to detect these bacteria,” Dr. Jones warned.

Even if a mirror cell only escaped into a river or the soil, it could wreak ecological havoc. Viruses would be unable to infect it. Amoebae and other predators would find it indigestible.

Unchecked, mirror cells could come to dominate entire ecosystems. “The impact on the food chain would be devastating,” said Deepa Agashe, an evolutionary biologist at the National Center for Biological Sciences at Bengaluru, formerly Bangalore, in India.

What makes a mirror cell even more dangerous is that it will be mutating as it replicates, giving it the potential to evolve into an even graver threat.

“Then all bets are off,” said Ruslan Medzhitov, an immunologist at Yale University. “You can’t predict what will happen.”

As a result of these conversations, Dr. Adamala and her colleagues decided to abandon their work on mirror cells. “We’re saying, ‘We’re not going to do it,’” she said.

How to prevent others from doing it is an open question, one that the scientists plan to address at larger meetings in 2025. “It is important that before the beast is in our face, we have a chance to think through it collectively,” Dr. Agashe said.

“I share their view that the risks of creating mirror bacteria cannot be justified by the relatively limited potential benefits, and that mirror bacteria and other mirror organisms should not be created,” said Filippa Lentzos, a biosecurity expert at King’s College London who was not involved in the project.

Sharing these conclusions with the public and calling for a broad discussion, she said, “is a role model of responsible science today.”

Carl Zimmer covers news about science for The Times and writes the Origins column.

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