快猫短视频

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LOS ANGELES, 2025. Two police detectives gaze at a body sprawled on the floor of a seedy hotel room. One scans test results on a tablet screen. 鈥淣o prints or DNA. If this was our guy, he was careful to clean up.鈥� Her partner sweeps a microbe sensor through the musty air. The readout pings: a match. 鈥淚t was him. Six hours ago. Looks like he moved back in with his girlfriend.鈥� A grunt. 鈥淎nd they got a new dachshund.鈥�

Like our hapless suspect, we all leave traces of our microbes behind. We are haloed by an invisible nebula of bacteria, fungi and viruses. It鈥檚 inevitable that this gets transferred onto the things we touch, the people we meet, even the air we pass through. It鈥檚 our vaporous calling card. Recently we have started learning to unravel its message, and the results look set to change forensic science and policing for good.

鈥淭he distinction is a bit like the difference between gin and whisky鈥�

Your internal microbial community weighs about the same as your head and is unique. That much we have known for years. Then we discovered that the balance of microbes can affect your health. A technique known as a faecal transplant for instance has moved into the mainstream in recent years. Here, patients get a dose of a volunteer鈥檚 excrement suffused with a healthy balance of gut microbes to counteract intestinal complaints. But what happens when the critters that make up your microbiome waft away from your body was unknown.

Now a picture is emerging. We all have our own individual, identifiable ecosystem on our skin and it seems that it is this that sloughs off to form most of our microbe cloud, otherwise known as the external microbiome. The species line-up morphs over time, but these changes are small compared with the differences between two people鈥檚 microbiomes.

The distinction is a bit like the difference between gin and whisky, says Edwin Steussy, a lawyer interested in the forensic implications of microbial clouds. Whiskies can vary in subtle ways, but you would never mistake any of them for gin. Even identical twins have skin microbe populations that are as different as a highland malt and a London dry.

Caught red-handed

These differences can be used to identify you, long after you touched something. Take the 2010 study in which a team at the University of Colorado, Boulder, out of a group of 270 from just the microbial signature they left on computer keyboards and mice, even after the devices had been sitting around for up to two weeks.

The same is true for mobile phones. In 2014, , then at the University of Oregon in Portland, and his team looked at a predefined selection of DNA chunks that act as a measure of microbial diversity on both the phones and the index fingers of the owners. They found that . Another study a year later showed that people could be to their phones and shoes by microbial communities.

But to a homicide detective, may be even more tantalising. Meadow and his team turned one of the university鈥檚 labs into a sanitised chamber, complete with CSI-style plastic-covered walls (see pictures, right). People sat alone in a chair for up to 4 hours as air filters and traps collected samples of the microbes given off by their bodies. To make comparisons fairer, each was given an identical outfit of clothes 鈥� tank top, shorts, flip flops 鈥� that had all been laundered together. The team sequenced the DNA and catalogued the genetic material collected (PeerJ, ).

鈥淚t worked so well we didn鈥檛 believe it,鈥� Meadow says. The presence of most people could be clearly detected simply from bacterial emissions into the air, and some could even be individually identified 鈥� the first time this has been shown. Some people gave off more microbial detritus than others, and the individual clouds became more different from each other the longer someone sat there. Another experiment showed comparable results after just 90 minutes. 鈥淚n science, you usually fail 10 times before it works once,鈥� Meadow says. 鈥淭hat hasn鈥檛 been the case in this field so far.鈥�

Microbial barcode

It鈥檚 not all plain sailing though. 鈥淭here are far fewer microbes floating around in the air than there are on the surface of skin or in the gut,鈥� says , who worked alongside Meadow at the University of Oregon. That makes sequencing a person鈥檚 cloud challenging, but as the technology improves it should be possible to do it faster, says Meadow.

All this means your skin microbiome 鈥� and by extension your personal microbe cloud 鈥� can act as an identifying barcode for you. But that鈥檚 not all. Other strands of research are showing that our microbiome retains an imprint of what we have been up to, where 鈥� and even with whom.

For instance, when we hang out with people, our clouds begin to rub off on each other, says of the University of California, San Diego, who studies the microbiomes of people who live together. Knight was surprised at how many microbes . 鈥淲e thought small children would be huge vectors for microbiome sharing among adults 鈥� that children bring you close together, microbially speaking,鈥� he says. 鈥淲e didn鈥檛 really find that with kids, but we did with dogs.鈥�

Family members who live together, particularly couples, shared more microbiota with each other, Knight found. The effect was most pronounced with skin microbes, especially if there was a dog in the house: dog owners shared more skin microbes with their pets than with other people鈥檚 animals.

Knight has also investigated how our microbes rub off on our homes. Quite easily, it seems. 鈥淲hen you move into a new house, you unpack your stuff 鈥� and your microbes too,鈥� he says. All this means that microbiome data could one day be used by detectives to tell where someone lives and with whom, and even whether or not they have pets. First, however, it will probably be used as supplementary evidence, a way to narrow down a field of suspects so investigators can collect more familiar evidence, such as DNA.

It鈥檚 a leap from the lab to the courtroom, Steussy says, but for microbial forensic science the jump is coming 鈥� and it will be a big one. 鈥淚t鈥檚 one of three or four times since I started studying law that I felt I鈥檓 on the edge of an infinite cliff,鈥� he says.

Because of that, Steussy recently teamed up with scientists and another lawyer to consider what the future might hold. That produced a report titled: Steussy certainly thinks the answer is yes.

Meadow, who now works at a microbiome sequencing firm called . DNA is the gold standard of forensic science but it can鈥檛 tell you where someone has been, who they have been with, or whether they have a dog or cat, he says. 鈥淏ut in the foreseeable future we will be able to find that information just by swabbing dust.鈥�

Two samples of one person鈥檚 microbiome will never match exactly, because the genomes of bacterial cells and viruses are constantly mutating as they reproduce. As a result, drawing links between microbiome samples has to be done by comparing the populations of millions of different microbes all at once and seeing how closely related they are in terms of abundance and DNA.

Part of the reason Steussy thinks the cloud forensic science revolution is imminent is that this sequence-and-compare approach, known as phylogenetic analysis, is already making appearances in courtrooms. Take for instance a case in which an alleged rapist had HIV and the person who was raped is now also infected. Prosecutors can take samples of the virus from both, plus other sexual partners, and compare the types of mutations present in the different strains to build a 鈥渇amily tree鈥� showing which samples are descended from which.

鈥淒NA can鈥檛 tell you if someone has a cat or dog or who they hang out with. But we will be able to get that info just by swabbing dust鈥�

This sort of evidence has already helped convict rapists, says Steussy, and in the early 2000s, it . Juan Maeso, who had hepatitis C, injected himself with morphine and then used the same needle to infect 275 patients. He was sentenced to 1933 years in prison. Around the same time, phylogenetic analysis in Louisiana of attempting to murder his mistress by injecting her with blood infected with HIV and hepatitis C.

Analysing microbe communities on surfaces is already a handy tool (see 鈥�Microbial CSI鈥�). And cloud forensics would operate in a similar way. The idea would be to create family trees for thousands of different microbes and use these to reveal many more connections. We might expect to spot fluctuations in someone鈥檚 microbe cloud that bear witness to their associates, family, clothes and perhaps even their route through a building to a crime scene. 鈥淚t鈥檚 an incredibly powerful tool that is going to turn everything upside down,鈥� Steussy says.

But using clouds to identify people and pin down their movements is still some way from being admissible as evidence in a courtroom. More studies on larger populations and in real-life environments will have to come first. And to identify which person was at a scene you would have to create and manage large databases to compare samples against, which raises privacy issues.

Borrowed microbes

There are also several fundamental questions that remain unanswered. 鈥淲e鈥檙e asking things like how far in space your microbiome extends,鈥� says Hickey. 鈥淚s it spread across the room, or does it stay close to you? Do some people take over the room?鈥�

Then there鈥檚 the issue of whether a database of people鈥檚 microbiomes would be useful, given that these communities are constantly in flux. It鈥檚 a legitimate worry, but there should be a core cadre of microbes that mark us out consistently. It will be a question of using statistics to get at this 鈥渜uiet signal鈥�, says Meadow. He thinks machine learning algorithms will help, as will sequencing entire microbe genomes rather than just characteristic sections.

Even when it does arrive in court, microbial cloud evidence won鈥檛 be immune to false positives or manipulation. The more familiar people become with the concept of microbe clouds, the greater the odds that someone will dream up a way to game the system. Someone might bathe in a caustic solution to kill off their microbes, wear another person鈥檚 clothing or spread a sample of their rival鈥檚 microbes across a crime scene.

Another risk is that juries might be dazzled by science when microbiome evidence is first used in court. Microbiome data could be misinterpreted, like DNA was, says Edward Imwinkelried of the University of California Davis School of Law. In the early days of DNA evidence, he says, a combination of faulty analyses and unfamiliar juries led to questionable testimony and problematic convictions.

Regardless of these hurdles, Steussy would bet 鈥渓ots of money鈥� the technique has already been used in cases of national security. 鈥淚n a legal framework you have to clear a burden of proof,鈥� he says. 鈥淏ut in a foreign country, trying to find someone on a list of targets 鈥� those kind of niceties don鈥檛 come in.鈥�

The rest of us, though, might have to wait a little longer before our microbial clouds get us into trouble.

This article appeared in print under the headline 鈥淎 cloud of distinction鈥�