City fungi are evolving to survive the heat - and possibly infect humans

Most people don’t think much about fungi unless it’s mold on bread or mushrooms on pizza. But behind the scenes, fungi – especially the microscopic kind – are everywhere.

And now, a new study suggests that fungi are changing in ways that could put us at risk. The researchers took a close look at how fungi living in city environments are adapting to heat.

The team found that fungi in warmer spots within the same city seemed to be toughening up against heat. That is concerning, since the one thing that keeps most fungi from infecting humans is our body heat.

Fungi and human health

Fungi, like molds and yeasts, usually can’t survive at 98.6°F – the average temperature of the human body. That’s what makes us naturally resistant to most fungal infections. But this protective line, known as the thermal barrier, might be thinning.

A small group of fungi already know how to cross it. One of the most dangerous is Candida auris. It first showed up in a human patient in 2009.

Since then, it’s been linked to thousands of infections each year in the U.S. alone. It doesn’t respond well to drugs, spreads easily, and kills a lot of people. What makes C. auris different is that it can tolerate higher heat.

Researchers suspect that C. auris didn’t always have this ability. It may have developed it over time, possibly due to global warming. As outdoor temperatures rise, fungi face pressure to adapt or die. Some are adapting – and fast.

Studying fungi from across the city

To test how fungi are handling heat in cities, researchers from the Bloomberg School of Public Health at Johns Hopkins set out in Baltimore with candy.

Yes, actual sticky candy. It turns out that taffy-like candy works well to pick up tiny microbes from city sidewalks.

The team picked four areas in the city: one hot, one above-average, one average, and one cool. Then they checked how fungi from each site handled heat stress. What they found was unsettling.

Fungi from the hottest spots had less pigmentation. That may sound like a small thing, but it could mean a lot.

In cooler environments, fungi often make darker pigments – like melanin – to soak up heat. But in warm spots, less pigment may help them avoid overheating. It’s a sign of possible temperature adaptation.

Fungi is adapting quickly

Back in the lab, the team exposed the fungi to high heat – above 131°F – and tracked how well they survived.

The fungi from warmer areas didn’t just survive better. They also showed stronger heat resistance, even when they belonged to the exact same species as the ones from cooler spots.

One type of yeast they found, Rhodotorula mucilaginosa, isn’t usually a big threat to people. But the sample taken from the hottest site handled heat much better than its cooler counterparts.

Even more surprising, a sample of Cystobasidium minutum taken from a sidewalk heated to 101°F grew just fine at human body temperature. That’s not common for this species, and it suggests it’s adapting to thrive in hotter conditions.

What this means for the future

“This study opens the door for future research into these adaptations and the identification of urban fungal species that may emerge as potential human pathogens in the near future,” said Daniel Smith, a postdoctoral researcher at the Bloomberg School.

Smith chose the sampling locations using temperature data from the National Oceanic and Atmospheric Administration. He also confirmed temperatures by measuring the sidewalk and dirt temperatures himself during collection.

The results suggest that urban fungi might already be inching closer to overcoming the body’s natural heat barrier. That’s concerning because it means that fungi we used to ignore may one day cause infections we can’t easily treat.

“These findings are consistent with the idea that high temperatures in an urban environment can induce fungal heat adaptations, thus narrowing the thermal barrier to human infection,” said Arturo Casadevall, the study’s senior author.

The evolution of city fungi

Right now, the findings are early and based on one city. The team emphasized that more studies in different cities are needed.

The researchers also noted several possible variables they couldn’t fully control, like sunlight exposure, foot traffic, and animals.

“These data are preliminary, and we need more and larger studies of this kind to help us understand how fungi are adapting to rising temperatures,” Casadevall said.

Even so, this study raises real questions. As cities get hotter due to climate change and urban heat islands, fungi are quietly adjusting.

If more species evolve to withstand human body heat, the result could be a new wave of fungal infections – ones we may not be prepared to handle.

The full study was published in the journal ISME Communications.

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