Could global warming promote the mutation of fungi and give them the ability to infect and “take control” of human beings? This is the terrifying scenario imagined by the postapocalyptic series The Last of Usbroadcast on the American channel HBO and Amazon Prime, in which a fungal epidemic decimates humanity, leaving only a few survivors – the last of us – facing hordes of “zombified” humans.
If it is a fiction, the authors of the series and the eponymous video game were inspired by a very real fungus, the cordyceps, capable of infecting insects and even modifying their behavior. They also have fun adorning their work with a scientific veneer, since the first episode features two researchers, one of whom evokes, looking serious and serious like all fictional scientists, the hypothesis of a global pandemic. What arouse the curiosity of the spectators.
“Cordyceps is very fragile”
At the Botany, Phytochemistry and Mycology Laboratory of the University of Montpellier, French scientists study all kinds of fungi, including cordyceps, which they cultivate on their premises. Looking less serious but just as serious, they first sweep away the hypothesis of the series which is, if not impossible, very improbable.
“Global cooling would favor this fungus more than global warming, begins Luis Portillo, teacher-researcher and cordyceps specialist. 18°C”. While there are variations – cordyceps can grow at 14°C – one thing is certain: above 30°C they pause or even wither away. “We unfortunately experienced it in our cultures last summer, when the air conditioning in the laboratory broke down,” he laments. For the same reason, it does not represent a danger for humans, since our internal temperature is 37°C on average. As for the hypothesis of a mutation that would allow it to adapt to the strongest heat, the researchers point out that this fungus has existed for 48 million years and that it has not radically mutated. Nor has it eradicated the insects they attack, such as ants.
Luis Portillo, in front of a culture of cordyceps, in the Ater laboratory, in Montpellier.
© / Botany, Phytochemistry and Mycology Laboratory (University of Montpellier)
Among the other inaccuracies in the series, his colleague Sylvie Morel, teacher-researcher in mycology, points out that “cordyceps never infects its victims thanks to the mycelia, but only with its spores”. The researchers add that cordyceps is a “very fragile” organism, particularly sensitive to fungicides, these synthetic products used to eradicate fungi.
“We feed the four species grown in our laboratory with rice, but that from conventional agriculture is not suitable for them, because it contains residues of fungicides that kill it. It’s the same with “organic” rice. who have obtained exemptions for fungicide treatments. We have to buy organic rice in which there are small critters, so we are sure that it has not been treated!”, explain the two specialists. The epidemic that broke out in Indonesia is thus not very credible, since this country practices the aerial application of fungicides on a large scale in order to fight against a very resistant fungus which attacks oil palm crops. Result: there are probably no more wild cordyceps in Indonesia.
The process of contamination still poorly understood
The hypothesis of a human pandemic should therefore remain a fiction for the moment. But for ants, caterpillars, wasps, etc., its infection is a cruel reality. Some species of cordyceps even manage to “take control” of their host and promote behaviors that push them to infect their congeners. A study appeared in Proceedings of the National Academy of Sciences in 2017 indicates that the Cordyceps unilateralis is able to grow throughout the entire body of ants, particularly targeting its muscles, but sparing the brain. Then it weaves a vast nutrient exchange network that communicates through the ant’s body. The latter then goes to wet areas – favorable to the development of the fungus – or climbs to the top of plants and clings to them with its mandibles… Until its death. The reproductive structure of the cordyceps then emerges from its body, produces a fruiting body which spreads spores on the congeners below.
Does the cordyceps possess, then, a form of intelligence? “It’s not about intelligence, but about adapting to the environment, sweeps away Luis Portillo. Cordyceps unilateralis manipulate the ants so finely that you might think he’s intelligent, but in reality it’s only the individuals who managed to get the ants to infect others who survived, because not all of them succeed. It’s a mechanical process of natural selection.” If the scientists have not yet solved the mystery of this change in behavior, they put forward two hypotheses. Either the nutrient network of the cordyceps produces molecules that modify the brain at a distance, or it takes direct control of the ant’s muscles and pushes it to move towards key areas.
Luis Portillo and Sylvie Morel favor the first hypothesis, supported by a study published in 2019 in Fungal Ecology laying on the fungus Massospora cicadina which attacks cicadas. “This study shows that this mushroom produces psilocybin, a psychedelic substance that alters the state of consciousness of the insect, but also cathinone, an amphetamine that gives it an excessive sexual appetite, describe the researchers. This does not is therefore not the fungus that tells the cicada to go here or elsewhere, but molecules that modify its behavior”. As a result, infected cicadas turn into hyperactive, hypersexual zombies that travel up to ten times the normal distance of a cicada. Which allows to Massospora cicadina to spread as widely as possible.
A similar mechanism is at work in Chordodes formosanus, a worm that parasitizes praying mantises. Since it has to change hosts between each stage of its development, it produces molecules that simulate thirst in the host cicadas, which then rush to watering holes, where the worm can come out and wait for its next victim. “I think that Cordyceps unilateralis works the same way: it must produce chemical compounds simulating thirst, heat or lack of oxygen, which would push the ants to move into humid areas or climb to the tops of plants”, suggests Luis Portillo.
In order to prove this, further studies will have to be carried out. Researchers at the Montpellier laboratory mainly study mushrooms in the hope of finding new molecules with pharmaceutical potential. “As much as there are many scientific studies on plants and their molecules useful for the creation of drugs, there are very few scientific studies on mushrooms”, points out Sylvie Morel. However, fungi constitute gigantic reservoirs of chemical molecules.
However, the matter is not simple. “Even if we find an interesting molecule, it must be produced by a fungus resistant, in particular to fungicides, which can be cultivated in a sustainable way”, she continues. The two researchers specify that their team is currently studying an anti-inflammatory property of a cordyceps, with encouraging results. If they are confirmed, the cordyceps will gain its letters of nobility as a reservoir of drugs, rather than a large-scale zombie potential. Phew!