In the computer game and science-fiction series The Last of Us, humanity is decimated by a Cordyceps fungus pandemic. This fungus mutates and begins to attack the human nervous system, turning the infected into aggressive, zombie-like creatures. Although fungi that turn people into monsters are obviously a work of fiction, the authors of this idea were inspired by the interesting life cycle of real fungi.

Dr. Biedunkiewicz from the University of Warmia and Mazury in Olsztyn told  PAP that Cordyceps fungi are in fact parasites of invertebrates. They attack some insects, also in the pupal or caterpillar stage.

The most imagination-provoking parasitic species is Ophiocordyceps unilateralis, which attacks ants. Insects infected with this fungus actually behave strangely. In order not to infect the family, they move away from the anthill - where the conditions are unfavourable for the development of the fungus - and start a solitary lifestyle, unusual for ants. At some point, the infected ant climbs a plant, and in a place where the conditions for the development of the fungus and spore dispersal are very good, the insect sticks its mandibles into the leaf and remains there. After the death of the ant, fruit bodies grow from its head (their shape resembles a club, hence the name of the fungus that roughly translates to 'club-headed') and spores are dispersed after their maturation.

According to the mycologist, the fungus does not attack the nervous system, and therefore the brain of ants, but their muscular system (especially legs), so it can, for example, use toxins to block their ability to move at the right moment. Ophiocordyceps unilateralis can also change the way ants react to stimuli (hence the search for high places to climb in order to increase the spore dispersal range).

Biedunkiewicz said: ’These fungi have become so specialized in the course of evolution that they infect only two species of ants. There is no possibility for these fungi to now reverse their development and reorient their preferences to change the host.”

She added that humans are evolutionarily very distant from ants. In her opinion, in a mammalian organism these fungi would not have, for example, the chemical compounds they need to live. Not to mention that the parasites would not be able to effectively control the behaviour of the new host.

Fungi that could become dangerous to humans, however, are black yeast-like fungi of the genus Exophiala. They live, for example, on kitchen and bathroom gaskets, plugs and sink fillers. 

Biedunkiewicz said: “They are dangerous because they are difficult to combat: they are resistant to chemical agents and high temperature.”

When asked if we are really threatened with pandemics of any fungal diseases, the mycologist said: “Mass fungal infections are not common. And these diseases develop quite slowly compared to viral diseases, for example."

However, she added that the treatment of fungal infections is difficult and long. Fungi are able to pass information to each other, just like bacteria. This is called quorum sensing. They can acquire resistance to drugs or new chemotherapeutics from each other.

In addition, fungi quickly adapt to new conditions, they are extremely flexible, they are not killed by high pressure or temperature, they also quickly adapt to anaerobic conditions. There are species that are able to survive at low temperatures, but there are also those that multiply well at the temperature of the human body, and also those that are not destroyed by high temperatures. That makes them difficult opponents to fight. In addition, in new habitat conditions, they produce adaptive enzymes that make it easier to conquer these ecological niches.

Although we already have antifungal therapies, the treatment is complicated - it requires combining agents that will destroy the fungal cell wall (to open access to the inside of the cell) with substances that will then damage the pathogen's DNA. 

Biedunkiewicz said: “While the treatment of bacterial diseases with antibiotics can take, for example, a week, fungal diseases take months to treat.

“And yet, we are surrounded by fungi. We eat them not only in the form of forest mushroom dishes, but - much more often - in bread, in the form of yeast. Many species of fungi also inhabit our digestive system, where, among other things, they take part in digestive processes or the production of vitamins. 

“And in one breath, we inhale about 10 fungal spores into the lungs.”

She added that the body is able to recognize some agents, such as fungal cells in the respiratory tract, and mechanically remove them using the sneeze and cough reflex. And this is just one of the methods the immune system has to defend itself against the adverse effects of fungi until the body's immune barriers are broken.

But in people with weaker immune systems, fungi can cause serious problems.

As decomposers (organisms at the final stage of the food chain; they feed, among others, on the remains of other organisms), fungal microorganisms compete with bacteria for an ecological niche. They develop when a place is freed up in a suitable habitat, e.g. by bacteria - and this happens, for example, in the body during aggressive antibiotic therapy.

Biedunkiewicz continued: “So it may happen that in a person who becomes infected with a fungus in a hospital, mycosis - and not the main disease - will become the direct cause of death. That is why patients who have been treated for a long time for bacterial infections, should also receive antifungal drugs as a protective measure.”

She added that even fungi that normally coexist peacefully with humans can cause a disease when they multiply too much. Even yeast - baker's yeast or yeast used in yeast treatments - can contribute to a yeast infection, the researcher points out.

She said: “This does not change the fact that fungi surround us and we cannot completely eliminate them from the environment. In order to defend ourselves against fungi that can be harmful to us, we need to take care of our immune system. Get enough sleep, avoid stress, eat healthy and varied food. Then the body is more resistant - also to fungi.”

When asked about the opening titles of the series The Last of Us, she pointed out that the yellow organism shown there is a slime mould in the plasmodium stage, which is not really a fungus, but a fungus-like organism. 

PAP - Science in Poland, Ludwika Tomala

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