A 5,300-year-old corpse, preserved in ice, still harbors living microorganisms. This is the troubling discovery that researchers have just made about Ötzi, this natural mummy discovered in 1991 in the Italian Alps. The team analyzed samples taken from the body, from inside it, as well as from the cold room where it is stored.
The goal was simple: distinguish the microbes that lived in Ötzi's gut during his lifetime from those that settled after his death, either in the glacier or in the museum. To do this, the scientists compared ancient, highly degraded DNA with that of more recent microbes. A precision work that offers an unprecedented picture of the invisible world accompanying this mummy.
Ötzi was discovered in the Alps, at 10,531 feet (3,210 meters) above sea level. A microbial ecosystem five millennia old
The researchers found original intestinal bacteria, close to those of early human populations. These strains are now very rare among inhabitants of industrialized countries. Ötzi thus offers a unique view of our microbial past, before changes linked to modern diet and antibiotics.
Unexpectedly, cold-adapted yeasts were isolated on the skin, in internal meltwater, and in the stomach. Their DNA shows similarities to strains found in Antarctica. These microscopic fungi are therefore likely native to the glacial environment where Ötzi was found.
Some of these microbes may even still be active. The geneticists observed both highly degraded and well-preserved DNA. This indicates that these yeasts are not simply remnants of the past: they continue to exist, perhaps in a dormant state, under the temperature of 21 °F (-6 °C) and high humidity of the conservation chamber.
Microbiologist Mohamed Sarhan examines yeast colonies taken from a sample of Ötzi's stomach.
Credits: Eurac Research | Andrea De Giovanni Yeasts that raise questions for conservation
Three of the four identified yeasts possess a gene allowing them to break down phenol. This compound was used just after Ötzi's discovery to kill fungi on the mummy's surface. The yeasts may therefore have used this substance as a food source, raising questions about past conservation methods.
Today, conditions are very stable, with strict microbiological monitoring. The mummy shows no signs of degradation. But this study, published in the journal
Microbiome, shows that a delicate balance exists between preservation and microbial activity. The scientists call for continued research to protect Ötzi for future generations.
Beyond the mummy, these cold-adapted microorganisms could find industrial applications, for example in low-temperature fermentation processes that are less energy-intensive. A lead the researchers intend to explore.
Going further: how do we know a microbe is ancient?
DNA is a fragile molecule. Over time, it fragments under the effect of water, radiation, or temperature variations. The older a microbe, the shorter and more fragmented its genetic strands are. Scientists measure this length to estimate the relative age of a sample.
Beyond fragmentation, certain chemical bases transform spontaneously. For example, cytosine often becomes uracil, a modification rare in living organisms but frequent in ancient DNA. By detecting these specific changes, researchers can distinguish a microbe several millennia old from recent contamination.
These techniques were applied to Ötzi's yeasts. The researchers observed very short DNA and typical degradation signatures, proving that these microbes indeed accompanied the mummy for thousands of years. In parallel, the presence of intact DNA on the same samples suggests that some cells remained viable until today.
Can a yeast really stay alive for 5,000 years?
Yes, provided it enters a state of deep dormancy. Faced with cold, lack of nutrients, or desiccation, certain yeasts slow their metabolism until it almost completely stops. They no longer divide, no longer consume energy, but maintain their cellular integrity. This state, called cryptobiosis, allows extremely long survival.
In samples taken from Ötzi, researchers managed to cultivate yeasts in the laboratory. This means that these cells, which remained on the mummy for 5,300 years, were still able to wake up and multiply once placed in favorable conditions. The proof is formal: their viability has not been completely lost.
The constant cold of the glacier, then of the conservation chamber at 21 °F (-6 °C), played a preserving role. Cold-adapted yeasts possess antifreeze proteins that prevent crystal formation inside their cells. Thus protected, they can wait for millennia, watching for the return of a milder environment.
Author: Cédric DEPOND