Microbial clock telling time of death

We can learn a lot about a person based on their skeleton, but a non-human material may also be informative in providing clues about what happened to an individual. Forensic anthropologists are investigating if the microbes which live in and on our bodies could reveal crucial information about time of death, and maybe more. Jessica Metcalf, from Colorado state university, explains to Julia Ravey how death impacts our microbial roommates...

Jessica - After death, the environment for our human microbiome changes dramatically. For example, temperature is no longer regulated, the immune system's no longer active. And that allows for certain types of microbes to proliferate. Also, in the case of an outdoor death scene, you also have environmental microbes that can then invade the body that we don't usually have living on us.

Julia - How can we use these changes in the microbes on us and in us to help inform forensics?

Jessica - The really cool thing about this microbial decomposer community that assembles after death is it's actually really predictable and fairly consistent - they emerge sort of at the same timeframes. And so that allows us to use all these microbes to build a model that then can predict how long a person's been dead within a certain amount of error.

Julia - If you just found bones or remains, can we still use microbes to tell us about the death of this individual?

Jessica - The more days out from time of death, the fewer tools and types of evidence, usually you have for estimating time of death, unless you have something like a last sighting or a last text message or something really solid like that. When we look at the microbial community that forms on the skin or nearby soil, if it's outdoors, we're usually looking sort of the first 21 days. And then after that, one of the things we've been working on is looking in the bones, so the microbes that then are able to more slowly invade the bone as kind of that longer clock and that tends to work really well as well. The error is a little bit bigger, but it's still really useful, especially since there's really not anything else to help you in those timeframes.

Julia - And how do you study this? Because I'm guessing it's quite a difficult thing to study at crime scenes. So how have you managed to chart this microbial clock?

Jessica - In the United States, we have this really valuable resource of anthropological forensics facilities that accept willed body donations. So humans that have decided to contribute their body to specifically forensic science. And so we work with a number of these facilities to place cadavers out and then we sample daily, for example, for 21 days. And then we profile the microbes for each sample using DNA sequencing.

Julia - And do things like temperature impact this change in microbes over time?

Jessica - Yes. In fact, we don't use a chronological day. we use something called accumulated degree day when we are estimating the time of death. So when it's hotter, you have a lot higher accumulating degree days and in the winter slower, and that's because microbes, their activity is dependent on temperature.

Julia - Do you think this type of technology could be used in other ways in forensics? For example, if there was wrongdoing in a crime, could we look towards the microbes, almost like DNA, to see if there are certain microbes there that could tell us a little bit about maybe what happened to this individual?

Jessica - Yes, absolutely. I think this is the tip of the iceberg in how we can use microbes as physical evidence. Another area of research I've worked on is trace evidence. So each of us have an individual skin microbiome that's fairly consistent over time. And when we touch things, we transfer it. Looking at that as a potential type of evidence as well. One could imagine a person's gut microbiome could tell some stories about what that individual has been up to. Certainly there are people working on things like the dust microbiome and what is the biogeographical tag that might be contained in that type of sample. So I think there's a lot of different ways that we can potentially use microbes as physical evidence.