One of the better-known episodes of the original Star Trek series is called “Mirror, Mirror” and it dramatizes a freak transporter accident which sends Kirk, Uhura, Bones, and Scotty to an alternate universe. Their mirror universe colleagues on the Enterprise are greedy, violent, and bent on conquest. And Mirror Universe Spock sports a goatee, so we know he’s evil.
The scientists who penned an article last Christmas entitled are not worried about being replaced by their bloodthirsty, bearded counterparts. Rather, they want to start a discussion about the risks of creating artificial life forms, specifically bacteria, whose molecules are slightly different from ours.
They conclude by writing, “We therefore recommend that research with the goal of creating mirror bacteria not be permitted, and that funders make clear that they will not support such work.”
The alternative they imagine reads like a catastrophic science fiction novel: mirror bacteria evading our immune system, surviving our best antibiotics, and ravaging plant and animal life. A macroscopic apocalypse caused by microscopic bullets.
What exactly is mirror life and why are these scientists so scared?
Handy molecules
Since the mundane bacterium (plural “bacteria”) is the focus of their concern, let’s begin by describing how it functions. You can imagine a bacterium as being round or elongated. What’s important is that its insides are separated from the environment by a cell membrane, much like how a fence can delineate a piece of land. Inside the membrane, the bacterium’s DNA has its genes transcribed into RNA, which gets translated into proteins. This simple premise—DNA being copied into RNA, which serves as a template for proteins, all within a cell that is bound by a membrane—is key to life as we know it. But there is something fascinating about the molecules involved in this process, a peculiarity which was first demonstrated by Louis Pasteur.
When we look at DNA closely, we can see that its backbone is made of a sugar called deoxyribose. That sugar is a lot like our hands. Our left hand is a mirror image of our right hand. You can’t superimpose them. The deoxyribose found in the DNA of bacteria, the deoxyribose found in our own DNA—in fact, the deoxyribose found in the DNA of all known life forms is like our right hand. It’s in a configuration scientists refer to as “D,” from the Greek dextro meaning “right.” DNA has a backbone of D-deoxyribose. In theory, there is an L-deoxyribose (from the Greek levo, “left”) but we don’t see it in nature.
Similarly, the amino acids that are strung together inside of cells to create proteins have mirror images (except for glycine, whose mirror image is the same). The ones we find in nature are the L-amino acids.
Because of all of this, we can say that life on Earth is chiral (pronounced with a hard “k” sound at the beginning). The word “chiral” is derived from the Greek word for “hand.” Life at the molecular level has a handedness. We have not found mirror life—made up of L-deoxyribose and D-amino acids—out in nature. But what if we could make it in the laboratory?
The helping hand of mirror drugs
Mirror DNA and mirror proteins are already being synthesized in laboratories around the world, and you may wonder why. Some will point to mountaineer George Mallory, who is alleged to have said to a reporter who wondered why he wanted to climb Mount Everest, “Because it’s there.” Scientists are often driven by their curiosity. And sure enough, basic research with no obvious application should not be discarded. After all, it was curiosity that gave us Taq polymerase, an enzyme made by bacteria that can survive the hot temperatures found in hot springs. This precious enzyme is now one of the workhorses of molecular biology, allowing scientists to copy stretches of DNA in the laboratory.
Yet, mirror proteins, even now, have potential applications. Many of the newer drugs available to us are proteins or shorter chains of amino acids known as peptides: for example, semaglutide (known as Ozempic or Wegovy) for diabetes and weight loss, bevacizumab for certain cancers, and many biologics that target autoimmune diseases like psoriasis and rheumatoid arthritis. One of the problems with these drugs is that they can be recognized and degraded by enzymes inside our body, which limits how long they stick around. Scientists are now testing proteins with some of their amino acids replaced by their mirror images. The goal is to see if protein-based drugs can be made as efficacious by incorporating mirror amino acids yet end up being more stable in the body.
The field of synthetic biology—where the molecules of life are created artificially in a lab—can feel like a mountain-climbing club. First you trek to the top of your local hill. Then you tackle a higher mountain, and an even higher mountain. One day, you plan on summitting Everest itself. Synthetic biologists have been creating ever longer strands of mirror DNA and mirror proteins in the lab. Could they one day synthesize an actual living bacterium, whose DNA and proteins and all chiral molecules are the mirror images of their natural counterparts? The interdisciplinary team which came together to write this recent warning about the risks of mirror life believe that this is possible within a few decades, or even a mere ten years if such a project were to receive the kind of funding and attention that the COVID-19 vaccines did. Artificially created mirror life is possible. It might just be a question of time and money.
While individual mirror molecules may not pose cataclysmic risks, the prospect of a complete mirror life form is more distressing.
Invisible invaders
What keeps us from being killed by disease-causing bacteria, apart from sanitation and antibiotics, is our immune system. Our body possesses a complex web of specialized cells, chemicals, and signalling molecules that act as a molecular army against microscopic threats. But here’s the twist: many of these immune soldiers depend on chirality to do their job. Much like how a southpaw boxer can surprise a new fighter who has only encountered right-handed opponents, a mirror bacterium could hold a distinct advantage over our immune system, to the point of being invisible to it. When we explain how molecules bind to the receptors at the surface of cells, we often describe it as a handshake, and this is even more true than it looks. Try shaking someone’s left hand with your right.
It is likely that mirror bacteria will not be detected by our immune system because their building blocks are symmetrically flipped, which means that they don’t fit in the receptors that have evolved to bind to them and signal a threat. And just like mirror proteins could escape from being destroyed by our enzymes and thus benefit medical treatment, the mirror proteins of mirror bacteria could linger in the body as well. While the mirror toxins they would produce might not work inside our body, the mere fact that they could grow, and grow, and grow inside of us could be danger enough, much like how a cancer disrupts our organs by expanding, and crushing, and stealing nutrients.
You may wonder, though, how mirror bacteria would even survive in our world. After all, most of our food contains chiral molecules that need to be digested by our own chiral molecules. But there are nutrients, like glycerol and butyric acid, that are achiral, meaning that these molecules do not have a left-handed or a right-handed version, much like how a circle is the same as its mirror reflection. These nutrients can feed both a normal bacterium and a mirror bacterium. Plus, it’s easy to imagine a scientist creating a genetic modification to allow a mirror bacterium the ability to process glucose from our world.
Mirror bacteria could be made in the lab using a bottom-up approach—where the DNA and the molecular machinery needed to translate DNA into working proteins are all synthesized in the lab and packaged inside of a membrane in the hope that, like a computer, it can be booted to life—or a top-down approach—in which a natural bacterium has its DNA reprogrammed to start spitting out mirror molecules until it becomes a mirror organism. Given that laboratory leaks do happen and that biological warfare is tempting to bad actors, containment of such mirror life is unlikely to be feasible when the technology exists.
Out in the wild, the natural predators of bacteria, such as phages and protists, would also probably not be able to shake the mirrored hands of these new bacteria and subsequently kill them. Mirror bacteria would be allowed to slowly multiply in the environment and disrupt both plant and animal life.
All of this—discussed briefly in the article linked to above and much more deeply in by the same authors—is a series of highly educated guesses. We don’t know everything there is to know about how life functions, but we know enough to be concerned at the prospect of creating mirror life. This is not wild speculation; rather, it’s about using our current scientific knowledge to try to predict best- and worst-case scenarios and to avoid the latter.
Their assessment is not without pushback. A response by Professor David Perrin, a bioorganic chemist at the University of British Columbia, was published in the same journal (accessible at the bottom, under “eLetters”). Perrin argues that the threat posed by hypothetical mirror microorganisms is likely to be less than the very real danger of drug-resistant bacteria and of viruses that make the jump from animal hosts to human hosts, like the COVID-19 coronavirus did. He emphasizes that if a mirror bacterium can evade our immune system, its own toxins are likely to be harmless to us, and that our antibiotic armoury could easily be recreated as mirror versions to combat these new microbes.
These discussions need to happen now. The risks outlined in this paper are major and, much like how scientists have refrained from recreating or modifying the smallpox virus because of the danger it presents, there is an argument to be made for green-lighting mirror molecules that have beneficial applications but for putting a stop to the development of entire mirror organisms. I wonder, though, if this line is even sufficient. What about mirror prions? Prions are proteins that replicate without the need for DNA and which infect other proteins, causing them to become misfolded and leading to infections like mad cow disease. Would a mirror prion remain invisible to the body? I don’t know. Yet another question to add to the pile, perhaps.
In the Star Trek episode “Mirror, Mirror,” our Starfleet heroes escape from the mirror world by working together. Hopefully, our own actions will see themselves reflected in this piece of science fiction: joining forces to properly assess the threat posed by mirror life and to prevent any disastrous consequences.
“We are hopeful,” the paper’s authors write, “that scientists and society at large will take a responsible approach to managing a technology that might pose unprecedented risks.”
Here’s hoping.
Take-home message:
- The sugar that makes up the backbone of DNA and the amino acids that get strung together to form proteins have a handedness, in the sense that mirror images of them have not been seen in nature but can be made in a laboratory
- Scientists are studying whether the kinds of proteins used as therapeutic drugs could stay in the body longer if some of their building blocks were replaced by mirror equivalents
- An interdisciplinary team of scientists has published a warning about the dangers posed by hypothetical mirror bacteria which could be made in a lab in the next few decades, as these bacteria would likely not be recognized by immune systems and pose a significant risk to human, plant, and animal life
