Meet the suite of microbes which decay dead bodies

Researchers in the United States have identified what appears to be a network of around twenty types of microbe that universally drive the decomposition of animal flesh. It’s thought the discovery could have huge implications for forensic medicine. And it was by studying the decomposition of human bodies in specialised facilities built for this purpose in America that led Jessica Metcalf, from Colorado State University, to identify these key microbes. She told Chris Smith about the work…

Jessica - In the United States we have these forensic anthropological facilities. These are places that definitely have really big fences around them, they're outdoors, and where people can choose to donate their bodies to be part of a forensic science; a sort of anthropology bone collection. We coordinated across three of these facilities to decompose 36 human donors over four seasons. We sampled these donors, who were placed outdoors to decompose daily for 21 days. These were swabs that we either rubbed a little bit on the skin or we also sampled the soil right near them. We were capturing on those samples the microbial communities that were active. Then we were able to get the DNA out of those and we used the DNA then to profile what microbes were there at that time. We built that up and we ended up with a time series of snapshots over 21 days for each of those 36 cadavers. That's a really powerful data set. It allowed us to see what microbes were there and what they were doing. Also, we looked at some of the small molecules they were making as they broke down things like fats and proteins that were coming from the cadavers.

Chris - Are you testing different environments and also different times of year? Because I would think that might make a difference to these outcomes, wouldn't it?

Jessica - We sampled bodies that were placed at three different locations that covered two climates, and then across all four seasons. What we found is that actually there's this core set of microbial decomposers that show up regardless of all of those variables. When we started looking into, well, what is the reservoir? Where are they hanging out? Where are they persisting when there's not something decomposing? We found no evidence for them being either gut or soil, which were our two big environments that we thought might be responsible. Maybe people were carrying them around. We also looked at these big data sets that are available on the human gut, human body and mini soils, and we really find very little evidence. There's one or two of these microbes that appear that you can find in the soils or associated with humans but, for the most part, the only place we found them were on insects. What we think, and this makes a lot of sense, if there's an animal, it could be a deer, it could be a bunny, whatever, decomposing in the forest, that particular location may not have had an animal decomposing on it for a thousand years. So how do these decomposers microbes persist? Well, they're hanging on to the blowflies and the carrion beetles that can move to visit bodies. To me, ecologically, it really made a lot of sense. Previously, we'd known that a couple of microbes that are involved in decomposition came in on blowflies, and now we know it's a lot more than a couple. The main, key decomposers, pretty much all of them are associated with insects.

Chris - That's fascinating because we always used to assume that your own bugs just declared war on you as soon as your immune system stopped when you died, that you were consumed by the army from within. What you're saying is, actually, that isn't the case and it's all an invading army that comes on the feet of flies.

Jessica - We have a study going on right now of indoor bodies and so we'll see, because it doesn't mean that these core decomposers are doing all the decomposition, but they're certainly a network that's doing the bulk of it and they're specialists at it, so they're really good. Some of your microbes could still be betraying you after death and going after your nutrients but, for the most part, no.

Chris - How does this change our understanding, forensically, in that case, given that you've now found this and you've found these species come in, you know the timeline, how does this enable us to have a better understanding of what goes on if someone is killed or dumped or whatever out in the wilderness?

Jessica - The cool thing is that because there's these microbes that are everywhere, we can build this fairly general model from which we can predict how long a person's been dead based on the microbes that you sample when the body's found. We can do that fairly accurately. This provides another tool in a death scene investigator's tool set which, depending on how long a body has been decomposing, there may not be many tools available to you. In some cases this could be the only one.