In the seconds after you breathe your last breath, the living dead take over. Now that your blood no longer flows, oxygen levels within your body plummet and degrading chemical processes start up, making your tissues more acidic. Some of your bacterial inhabitants relish the change and flourish, while others die off.

The shift in your internal chemistry also attracts insects, which land on your body and creep into your orifices to lay their eggs, bringing with them their own microbial hitchhikers. Several days later, these eggs hatch, and as the larvae begin to feast on your flesh they carry with them yet more microbes – as well as antimicrobial compounds that inhibit the growth of susceptible bugs.

In other words, you might be dead, but your body is more alive than ever.

Forensic scientists are now trying to harness this “necrobiome”. They believe the bugs living upon and within you could harbour vital clues about the circumstances of your demise, and how long you’ve been dead for. And it’s not just the dead that may reveal their secrets through their bugs: microbial fingerprints might also be used in rape or sexual assault cases, to provide police with vital leads about the identity of the assailant.

Unique bacteria

Though you might consider yourself a single entity, you are in fact comprised of billions of living organisms. Some of them will be the same as those carried by other people, but many of them will be unique to you; a montage of the places you’ve been, the people you’ve touched, the foods you’ve eaten – not to mention your unique physiological make-up which some bugs will find more habitable than others.

One of the first hints that these bacterial hitchhikers might be useful to forensic scientists came in 2010, when Rob Knight and his colleagues at the University of Boulder in Colorado discovered that unique bacterial fingerprints could be recovered from people’s computer keyboards.

In the five years since, interest in these microbes has flourished, thanks to a growing appreciation of their diversity – and the development of rapid DNA sequencing techniques to study them.

Jeffery Tomberlin at Texas A&M University in College Station is one of those investigating the potential of the necrobiome for estimating the time since someone’s death. “As humans, we are an ecosystem that’s held in equilibrium by our immune systems, but when we die it becomes a free-for-all,” he says. “Within seconds of death, our microbial communities begin to change and that information could be important in terms of determining when that person died, where they came from, or what’s happened to them.”

Predictable populations

Until now, the main way of establishing those kinds of facts was to study the colonisation of corpses by insects – a field called forensic entomology. But the approach has its drawbacks.

“Entomology is good at determining how long the insects have been on the body, but that doesn’t necessarily tell you anything about time of death; you can have scenarios where the body isn’t colonised for several days, depending on conditions and circumstances,” says Tomberlin. “But with the necrobiome we are talking about the microbes on you; they’re with you, and when you die it starts.”

The current goal is to identify a handful of core species, present on pretty much everyone, and whose numbers change in a predictable way when we die.

“Our hope is that we will eventually be able to fine-tune this microbial clock, so that we can predict time of death to within several hours, rather than plus or minus two to three days,” adds Eric Benbow at Michigan State University in East Lansing, who collaborates with Tomberlin. So far, they’ve identified five potential core species, and they are now trying to validate this with further swabs taken from recently deceased humans. 

Meanwhile, Silvana Tridico at Murdoch University in Australia has her sights set on the bugs of the living. In December she published a pilot study which suggested that the bacteria living on people’s pubic hair could serve as a unique fingerprint in sexual assault cases – even when the hairs themselves aren’t shed.

Hair profiles

Rapes and sexual assaults can be particularly challenging for forensic scientists seeking physical evidence to back up a victim’s story. Before the advent of DNA testing, traces of semen could be categorised by blood group, narrowing the range of possible contributors. DNA testing is far more effective – it can identify a single perpetrator – but offenders are getting wise to the technique. “Based on my experience of the past 10 years, most of the guys carrying out sexual assaults are now using condoms because of fears about DNA testing; they are also taking them with them,” Tridico says.  

In a minority of cases, pubic hairs from the perpetrator may be found on the victim’s body; these can be physically examined, and subjected to mitochondrial DNA testing. But other tools are sorely needed.

Tridico took samples of head hair and pubic hair from seven volunteers on three separate occasions, and used DNA sequencing to build up a profile of the microbes living on their surface. Scalp and pubic hair samples were collected at the start of the study and at two and five-month intervals thereafter. Although the bacteria on head hairs were quite transient, all of the volunteers harboured a relatively stable population of bacteria in their pubic hair that marked them out as unique. Male and female pubic hairs could also be easily distinguished from one another, as women tended to harbour Lactobacillus; a genus that likes to grow in the vagina.

Bacterial detectives

What’s more, there’s also a hint that your bacterial population can become imprinted on the pubic hair of your sexual partner. Among Tridico’s volunteers was a romantic couple. Although they had been asked to refrain from intimate contact in the days before sampling, Tridico deduced that they’d disobeyed her; she detected the woman’s profile on the man’s pubic hairs. “I said to her: ‘I think you’ve just messed up my research project’. She said: ‘But he’s been away for three months’,” Tridico recalls. The bacterial imprint remained, despite both parties having showered.

This could be a useful tool in rape cases, where no other physical evidence can be retrieved. “I don’t think that this would be a stand-alone thing that could put somebody away, but it might just point police in the right direction,” Tridico says.

All of those working in this emerging field say that more work is needed to properly validate the techniques and find out just how unique a person’s bacterial fingerprint really is. But they suspect the applications could be numerous. One of Tridico’s subjects harboured a bacterium which thrives in seawater – it turned out he was a keen swimmer who lived near the coast. Yet other research groups are investigating whether bacteria could be used to identify the geographical origin of unidentified human remains.  

Who knows what tales your bugs might tell about you.

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