Of mice and men

Jessica Metcalf is an early-career researcher who had just published a paper in eLife looking at whether the bacteria recoverable from the body of a decomposing mouse can be used forensically to predict the time since death. Since then she’s been appointed as an assistant professor at Colorado State University, and she’s moved on from mice to men, as she explained to Chris Smith...

Jessica: When we spoke last time 4 years ago, I had just published my first paper, looking at how repeatable the microbial succession during decomposition was, where we decomposed mice all of the same age, in the same lab, on the same soil type, and we sampled them during 8 time points and we found really it’s the exact same set of microbes at each time point for each mouse.

Chris: Why were you doing that?

Jessica: This was our first step in trying to understand whether microbial succession during decomposition is a repeatable enough and a predictable enough process to potentially be developed as a forensic tool to estimate how long a person had been dead. And that was the first paper that we published in eLife.

Chris: Where have you taken the research since? It’s obvious that things have gone in the right direction for you because you’ve achieved a significant university appointment and begun to publish more on this topic. So, how are you developing that initial eLife publication?

Jessica: So our first publication in eLife really showed us that if we control all variables that we see this really repeatable succession of microbes during decomposition, and so then this launched a number of other projects where we start focusing on different variables and start changing them, and ask the question, ‘do we still see this repeatable succession of microbes during decomposition?’ which is what we need if we’re going to be able to develop this into a forensic tool. In 2016, we published a paper in Science where we described two big sets of experiments. In the first one, we did a very similar mouse study as we did in the eLife paper, but we used 3 different soil types that had really different starting microbial communities. And there, we were using soil as a proxy for environment. If a person dies in Moab, Utah or in the mountains of Colorado, are they going to have the same succession of microbes or is it going to be completely different? And this helps us understand, is there a universal clock that we can develop to estimate the time since death or does it need to be regional? What we found there is that really, the soil type didn’t matter. We still get a very similar succession of microbes. And then the second part of that paper, we actually were able to work with a anthropological research facility down in Sam Houston State University where people donate their bodies to become part of a forensic science experiment after death. And so, we had the opportunity to decompose 4 human bodies – 2 in the spring and 2 in the winter. We sampled the skin in the soil over time and we again, asked the question: In this outdoor scenario, where there's rainfall events, there's insects, there's scavengers, do we still get a repeatable succession of microbes during decomposition? And what we found was yes, we do and it’s fairly impressive. That was a real shock because we went from a lab setting where we were controlling so many things to this outdoor setting, and we were still able to recover this pattern.

Chris: So what did you do? Did you just lay the corpse on the floor as though the person had died in situ and are they clothed, unclothed because that could make a difference too, couldn’t it?

Jessica: For the studies that we’ve done so far and that we have currently going right now, we’ve placed all the bodies in the exact same way. So we’re still controlling for that variable at the moment but at some point, we’ll probably test that as well. Everybody is outside, laid face up without clothes.

Chris: Now, why do you think that you see these results because the soil is such a strong driver normally for what's around and what it can support and sustain? Is it that the body is so nutrient-rich and so replete with its own microbial spectrum, that it just basically feeds and out-competes anything that could come in from the environment, and that’s why you see this very reproducible succession of microbes?

Jessica: What we found is that on average, most microbes appear to actually be coming from the soil. So you have to remember that soils have an incredible diversity of microbes and even though 2 soil samples may look very different overall as far as their microbial diversity, they still have some of the same microbes. And so, what I think is probably going on is that these are just really ubiquitous bacteria that are at a low abundance in a lot of places across earth, and they are very good at responding quickly to a new nutrient source. The body is this huge pulse of nutrients and there are microorganisms that have evolved to be really good and once they find that boom, they're growing like bonkers.