Lewis E. MacKenzie of the University of Strathclyde in the UK, together with Dominic J. Black and Robert Pal of Durham University, has proposed a way to detect candidates for hypothetical "mirror life" using the polarization of light emitted by labeling molecules that bind to DNA and cell membranes. In a peer-reviewed perspective paper published in the journal Royal Society Open Science in October 2026, they describe a test that cross-checks circularly polarized luminescence from multiple labels. The proposal builds on earlier work, including the identification of labeling molecules inside living cells. The paper proposes a testing approach; its detection sensitivity and false-positive rate have not yet been measured.
What is reversed in mirror life?
The left and right hands are mirror images of each other, and no matter how you rotate them, they cannot be perfectly superimposed. Molecules can have the same property, known as chirality. In ordinary organisms on Earth, the amino acids that make up proteins are mainly the L-form, while deoxyribose, the sugar in DNA, is the D-form. In hypothetical mirror life, the configurations of key biomolecules would be reversed relative to ordinary organisms, for example with D-amino acids in proteins and an L-form sugar in DNA.
Research on mirror-image molecules themselves is already advancing. In a study published in Nature Biotechnology in 2022, L-form DNA that binds to thrombin, a human enzyme involved in blood clotting, was selected. Short nucleic acids that bind to specific targets in this way are called aptamers. However, making a mirror-image molecule that binds to a target is technically very different from making a cell that replicates its genetic information and passes it through generations. The existence of self-replicating mirror life has not been confirmed at this time.
Still, methods for detecting mirror life are being considered because the configuration of molecules governs how organisms interact with each other. In a policy forum paper published in Science in 2024, Katarzyna P. Adamala and colleagues pointed out that if self-replicating mirror bacteria were created, they might evade immune recognition that depends on molecular configuration, as well as predation by some predators. Infection and spread in the environment are also potential risks predicted from such mechanisms. The authors oppose creating mirror bacteria, while stating that research on mirror-image molecules that does not aim to create self-replicating mirror life has scientific and medical value.
What can left-right differences in emission tell us?
Circularly polarized light is light in which the electric field rotates in the plane perpendicular to the direction of travel. The rotation can be left- or right-handed, and the phenomenon in which the intensities of left and right circularly polarized components in emitted light differ is called circularly polarized luminescence. In the proposed test, labeling molecules (probes) added to a sample are excited with light, and their emission is split into left and right circularly polarized components and measured.
The researchers propose two main ways of using labels. One is to use "achiral" dyes, which can be superimposed on their mirror images. Even such dyes can show a left-right bias in their emission when they bind to chiral structures such as DNA. The researchers propose examining how a dye's emission changes depending on differences in the DNA helical structure.
The other is to use a pair of labeling molecules that are mirror images of each other, known as enantiomers. If each binds to different targets or accumulates in different places within cells, differences in circularly polarized luminescence may make it possible to distinguish their distribution. However, the left-right difference in circular polarization is a measurement reflecting the environment and binding state of the labeling molecule, and it cannot by itself directly determine whether the sugar in DNA is the D-form or L-form. It is therefore necessary to investigate in advance what kind of emission the labels show when bound to each target.
Why a single label cannot identify mirror life
Even DNA with the ordinary D-form sugar can, under certain conditions, adopt a left-handed structure known as Z-DNA. The direction in which the DNA helix winds and the configuration of its constituent sugar are separate properties. For this reason, finding a dye that responds to a left-handed structure would not by itself be evidence of mirror life. The researchers propose experiments using Z-DNA to confirm how much the emission changes when the winding direction of ordinary DNA is reversed.
Cell membranes also have features that complicate discrimination. The typical membrane lipids of archaea have a glycerol backbone whose configuration is the opposite of that in bacteria and eukaryotes. The researchers therefore proposed using multiple labels that bind to different targets, such as DNA and cell membranes, and checking whether the direction and intensity of each label's circularly polarized luminescence match the combination expected for mirror life. However, in samples containing multiple organisms, emission from different cells may overlap and be detected together. If it can be confirmed that multiple labels are reacting within the same cell, the reliability of the determination can be increased.
Technologies already demonstrated with microscopes and compact cameras
Cell observation using circularly polarized luminescence has already been demonstrated experimentally. In 2022, Patrycja Stachelek and colleagues introduced two types of europium complexes, mirror images of each other, into living mouse-derived fibroblasts and observed them. In the study using a confocal microscope, they simultaneously acquired left and right circularly polarized images and distinguished the distribution of the labeling molecules within the cells. One complex gathered mainly in lysosomes and the other mainly in mitochondria, and differences in emission made it possible to map each location. The values reported in the paper, 126 nanometers laterally and 396 nanometers along the optical axis, are spatial resolutions based on the diffraction limit when using an objective lens with 63x magnification and a numerical aperture of 1.4.
Measurement technology for circularly polarized luminescence has spread beyond microscopes to compact imaging devices. In 2023, Davide F. De Rosa and colleagues built a prototype portable circularly polarized luminescence camera with no moving parts and imaged known luminescent materials. The perspective paper considers a testing approach that combines such existing technologies.
| Year / study | Sample / assumed target | Demonstration / proposed use |
|---|---|---|
| 2022: Stachelek et al., confocal microscope | Living mouse-derived fibroblasts, each loaded with one enantiomer of a europium complex | Simultaneously acquired left and right circularly polarized images to distinguish the distribution of labeling molecules within cells |
| 2023: De Rosa et al., portable camera | Known materials showing circularly polarized luminescence | Imaged circularly polarized luminescence with a compact prototype with no moving parts. Original paper |
| 2026: MacKenzie et al., perspective paper | Hypothetical mirror life | Proposed a primary screening test that cross-checks circularly polarized luminescence from multiple labels |
What has been demonstrated so far with confocal microscopy is the identification of luminescent probes inside ordinary living mouse cells, not the detection of mirror life itself. The perspective paper envisions a division of roles: first selecting promising labeling molecules through spectroscopic measurements, then confirming where emission occurs with a confocal microscope, and using a compact camera for primary screening in the field.
How far can the method be validated without creating mirror life?
The proposed test can be validated using ordinary organisms and non-self-replicating model samples, without creating self-replicating mirror life. For example, researchers can examine whether labeling molecules reach their intended sites and how strongly they react with ordinary organisms. How accurately circularly polarized luminescence can be distinguished depends not only on the size of the left-right bias but also on the number of photons that can be detected. Even if the bias is large, it is difficult to measure the left-right difference if the labeling molecule does not absorb enough excitation light or if its emission is weak relative to the light it absorbs. In addition, samples containing a mixture of microorganisms and organic matter increase background signal, and cloudiness in a sample may cause the light to lose its polarization properties.
Such experiments can evaluate how well labeling molecules and measurement devices function under known conditions. The main indicators for judging test performance are sensitivity, the proportion of true targets correctly identified as positive; specificity, the proportion of ordinary samples correctly identified as negative; and the limit of detection, the minimum amount required for detection. However, performance measured with model samples is an evaluation of that model and those experimental conditions.
Also, if actual mirror life were extremely rare, even misidentifying only a small fraction of ordinary samples as positive could mean that most detected candidates are false positives. For this reason, when the primary screening finds a candidate, confirmation using a principle different from circularly polarized luminescence is important. One possibility is chiral analysis, which directly examines molecular configuration to verify whether molecular structures characteristic of mirror life are present. A key measure of the proposed method's practicality will be whether circularly polarized luminescence can narrow down candidates from mixed samples and lead to follow-up testing with specialized analytical equipment.