The ecological clean-up crew that breaks down dead bodies features several of the same species, and tend to always follow the same routine – regardless of location, climate or season, new research shows.
“These findings may contribute to society by providing potential for a new forensic tool,” write the study authors, led by microbial ecologists Jessica Metcalf and Zachary Burcham from Colorado State University.
Decomposers – mainly bacteria and fungi – are trying to eat us all the time, but our immune system, skin barrier, hygiene practices and beneficial microbiome usually kick them out – at least, while we’re still alive.
Of course, you can’t just leave human bodies lying around anywhere. This kind of research unfolds at ‘body farms’ (aka human decomposition facilities), where postmortem investigations can be done in relative peace, without anyone tripping over a deceased donor.
In this case, the researchers laid 36 human bodies out in the elements, each body being donated to science by individual before their death.
Each body was fresh (not frozen) and was in the very early stages of decomposition before they were fully exposed to all the weather and insects the environment had to offer.
The bodies were placed across three different body farms: one in the semi-arid, cold steppe of Colorado, and two in temperate regions, Southeast Texas and Tennessee.
Three cadavers were placed within each of these locations, for each of the four seasons. The scientists then tracked their decomposition – and the decomposers responsible – over 21 days.
At first, the decomposer community is pretty random and opportunistic – a spore blows up the nostril; a bacteria stumbles upon the unexplored crater of the anus; another is washed up on the shores of a recently exposed abdominal cavity.
But after a while, these communities of new arrivals begin to follow a clear pattern, and a consistent structure emerges in this newly founded ecosystem – regardless of where the body lies.
“These processes led to a decomposer network consisting of phylogenetically unique taxa emerging, regardless of season, location and climate, to synergistically break down organic matter,” the authors write.
The climate and location had an impact on how fast the body breaks down, but in terms of who breaks the body down, these factors don’t seem to make much difference.
Using DNA samples from the bodies, their ‘necrobiomes‘ and the soil around them, the scientists mapped out the network of interactions that show how these organisms recycle all our bits and pieces – in a kind of production line of co-dependent digestion that might explain why these same species turn up side-by-side in so many different circumstances.
For instance, the fungi Candida and Yarrowia help break down lipids and proteins into simpler compounds, like fatty acids and amino acids, and usually occur alongside the bacterium Oblitimonas alkaliphila, which eats exactly the stuff these fungi ooze.
“We suspect that key network microbial decomposers are probably not specific to decomposition of human cadavers and are, in part, maintained or seeded by insects,” the authors write.
While they might not be specific to humans, these microbes don’t just turn up anywhere: they’re rare in environments without decomposition, and the researchers think they might only thrive in this grisliest of environments – a carcass laid out to rot.
Most of those ‘key network decomposers’ – both bacteria and fungi – found within the cadavers are known to be carried by blow flies and carrion beetles. And they’re the same crew involved in the terrestrial break-down of other carcasses: swine, cattle, and mice. Some of them have also been detected in aquatic decomposition.
Pairing the microbial data with machine learning, the researchers were able to accurately predict time since death for the cadavers, especially when using the skin decomposer microbes as a reference point. They hope this will be invaluable in forensic investigations in future.
This research has been published in Nature Microbiology.
