Bacteria and forensic science
Forensic scientists have range of techniques to work out how long a person has been dead. These include looking at things like insects that may have moved in to help decomposition along. Unfortunately, these measures are fraught with difficulties and they tend to be quite inaccurate. But now, University of Boulder, Colorado, scientist Jess Metcalf has been looking at another potentially much more accurate way of doing it. She's looking at the communities of microorganisms that grow in and on the body...
Jess - So, we know microbes play an important role in decomposition, that's no secret. Forensic scientists have known this for a long time. However, we haven't had the tools to really study the microbes and because decomposition is driven by biological processes, forensic scientists try to use biological information to estimate the time since death. For example, if you watch CSI or some similar program, you might always see a forensic scientist sampling blue fly larvae. Oh, it's at this stage this means that this body could have only been dead for so long and so our question was, can we use microbes to try to answer the same type of question? Because every approach in forensic science for estimating the time since death, they all have some amount of uncertainty around them, and if you combine these different approaches, you can narrow that window of uncertainty. And our question is, can we use the information about microbial community change during decomposition to help narrow this window?
Chris - Why should that be better than what the other measures can already offer, Jess?
Jess - Every type of approach has its limitations. So, for example, body temperature and really, that's only going to be relevant in the first 48 hours. And then things like the succession of insects might be limited by the seasons. So, sometimes in the winter, insects never show up at a body. So, you can't use that. However, we all have microbes in us and on us when we're alive, and so, we already have the organisms at the time of death.
Chris - It's like our own an in-built biomarker. So, how did you do this?
Jess - We used a mouse model system. And the reason we did that is that because really, what this question is about is if you have lots of replicates. Are you going to see the same microbial community change in each replicate or is it some stochastic response of the microbial community? What we want it to be, to be able to use it as sort of a biomarker to estimate the time since death is, we want it to be consistent across replicates. And that's really hard to do in using, for example, donated human corpses or even, what's often used in forensic sciences, is a swine model. Those experiments are usually limited to one to three individuals. And so, with a model system, like a mouse model system, we were able to use 40 individuals and we sampled 5 at a time over 8 time points. And this was the first time, someone was able to characterize that bacterial community change.
Chris - Could you also look, not just in the body, but in the environment because the body, by virtue of breaking down, is going to release various chemicals and nutrients into the surrounding environment, that which will in turn, surely alter the microbial communities there too?
Jess - That's a great question. And that's one of the things that we looked at in this study - how decomposition of a corpse affects the surround soil. Often times, you know, in a crime scene, you'll find a body on the soil outside and so, if someone moves a body, can we test the soil and say that something was decomposing on it? And so, for the soil, we're kind of interested in two different questions. One, can we also use the change in soil microbes to estimate the time since death? But also, is there a really significant signature left by the decomposition process? And the answer to both of those questions was, yes. Then at every site, we saw really significant change, in both the bacteria and sort of the nematodes and other microbial eukaryotes that were present. Everything changed very significantly.
Chris - How were you measuring the microbes? Was this using a DNA technique?
Jess - Exactly. So, what we did is we borrowed the techniques that are now more commonly used to really understand the human gut microbiome and take a sample at the depths of like millions of sequences, then characterized the community of bacterial species and the community of, say fungi and nematodes that are present.
Chris - But obviously, where the majority of the bugs are in a human is inside us, in our guts. And so, that's going to really strongly dictate what we then get rotted down by, at least initially, isn't it? So, does it make a difference to your assay do you think, or will it, whether you got a person from one culture or another culture, one sex or a different sex, because we all know that our microbial spectrum is as unique as a fingerprint? (We're not all)(12:15) you genetically identical study mice. So, will it still work in a highly genetically diverse and microbiologically diverse human population?
Jess - That's a really good question, and that's what our follow up experiments are getting at. Now, we're going to look across different host species. So, obviously, mice have different gut microbes than pigs or swine than do humans, and so, one of the things we're going to do with all our different studies is then compare across different host organisms or species. Does that influence what your decomposer community looks like? And that sort of gets at the same question of like, different diets and different sort of gut microbiomes.