According to JOHNS HOPKINS BLOOMBERG SCHOOL OF PUBLIC HEALTH
I. The Unseen Heat Island: Fungi Adapt to Urban Life
Cities are not just concrete jungles; they are ecological hot zones, experiencing a phenomenon known as the Urban Heat Island (UHI) effect. Buildings, roads, and reduced greenery trap heat, making urban microclimates significantly warmer than surrounding rural areas.
While we often track the adaptation of visible wildlife, new research from Johns Hopkins University (JHU) reveals a critical, hidden development: urban fungi are thermally adapting to these hotter conditions.
This study underscores a compelling, yet concerning, biological reality: the constant, elevated temperatures in city environments are acting as a powerful evolutionary force, selecting for fungal strains that are more heat-tolerant. Since fungi are ubiquitous and some species are human pathogens, this adaptation is not merely an academic curiosity—it represents a potential new threat to public health.
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II. The Selection Process: Survival of the Hottest Fungi
The JHU researchers studied various fungal species collected from different urban and rural environments. Their primary finding was clear: fungi isolated from urban settings consistently displayed a higher thermal tolerance compared to their rural counterparts.
The Urban Filter: The constant high temperature in cities acts as a selective filter. Fungal strains that cannot withstand the chronic warmth die out, while strains possessing natural or acquired resistance to heat thrive and reproduce. Over time, this process leads to a fungal population with a significantly higher maximum growth temperature.
Elevated Growth Temperature: The research demonstrated that urban strains maintained robust growth at temperatures that would significantly inhibit or even kill rural strains of the same species.
This adaptation is a direct consequence of the UHI effect and highlights how human-altered environments are inadvertently shaping the fundamental biology of microbes.
III. The Direct Implications for Human Health
The most pressing concern arising from this thermal adaptation relates directly to human vulnerability. The normal human body temperature (37 °C or 98.6 °F) is often the body’s primary defense against many fungal invaders.
Breaking the Thermal Barrier: Fungi typically struggle to grow optimally at 37 °C—this is a natural thermal barrier that limits their ability to establish a successful infection in the human host.
Evolving Pathogenicity: By adapting to the higher, sustained temperatures found in urban environments, these fungal strains are effectively lowering the thermal barrier they must overcome to survive inside the human body.
Increased Infection Risk: A fungus already comfortable growing at 35 °C or 36 °C is far more likely to proliferate and cause disease in a human host than a rural strain whose optimal growth temperature is much lower. This increases the potential for both systemic infections and the development of new fungal pathogens.
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IV. The Call for Monitoring and Intervention
The JHU findings reveal a hidden consequence of climate change and urbanization that must be integrated into public health planning. While the study did not focus on identifying specific new human-infecting species, it provides a crucial framework for understanding future microbial evolution.
Need for Surveillance: The research underscores the urgent need for a dedicated surveillance system to monitor the thermal tolerance of fungi, particularly those species known to cause infections (like Candida and Aspergillus). Urban environments, being the epicenter of this thermal adaptation, should be prioritized for microbial monitoring.
Rethinking Urban Design: The findings add significant weight to arguments for mitigating the UHI effect through urban planning—specifically by increasing green spaces, promoting reflective materials, and designing better ventilation systems. Reducing urban temperatures may slow the evolutionary selection process that is strengthening fungal pathogens.
The core viewpoint is that we are unwittingly strengthening potential adversaries. This thermal adaptation is a clear signal that the future of infectious disease surveillance must not only include viruses and bacteria but also the quietly evolving fungal kingdom.
Source: Wikimedia Commons — CC BY-SA 4.0
References
IPCC. (2023). Climate Change 2023: Impacts, Adaptation and Vulnerability.
According to JOHNS HOPKINS BLOOMBERG SCHOOL OF PUBLIC HEALTH
Key Takeaways
- Urban heat island effects are selecting for fungal strains with higher thermal tolerance, potentially producing populations of common environmental fungi that are more capable of infecting warm-blooded animals including humans.
- Research has documented shifts in the thermal optima of Candida and Aspergillus species isolated from urban environments compared to rural environments, consistent with temperature-driven selection.
- The hypothesis that climate warming drives emergence of new fungal pathogens (proposed by Casadevall et al., 2019) is supported by evidence that C. auris—a novel human pathogen—appears to have thermal tolerance not found in its closest evolutionary relatives.
- Urban birds and small mammals, which experience chronically elevated ambient temperatures, may serve as adapting hosts that bridge environmental fungi toward human-pathogenic thermal tolerance ranges.
- Monitoring the thermal biology of environmental fungal isolates from urban and rural areas over time represents an early warning system for emergence of thermally adapted fungal human pathogens.
Frequently Asked Questions
What is the ‘thermal exclusion’ hypothesis of fungal disease?
The thermal exclusion hypothesis (Casadevall et al., 2019) proposes that mammalian body temperature (approximately 37°C) serves as a significant physiological barrier to fungal infection—most environmental fungi cannot grow at 37°C and are therefore excluded from mammalian hosts by default. This thermal immunity complements immune system defenses and explains why invasive fungal infections are relatively rare in immunocompetent individuals despite constant environmental exposure to fungal spores. The hypothesis proposes that as global temperatures rise, some environmental fungi may be selected for higher thermal tolerance, potentially enabling them to overcome this thermal exclusion and infect previously resistant mammalian hosts. C. auris’s emergence in multiple continents simultaneously is cited as a potential example of climate-driven thermal adaptation.
How do urban heat islands affect fungal evolution?
Urban heat islands—areas where cities experience elevated temperatures (typically 1–5°C warmer than surrounding rural areas) due to heat-absorbing building materials, reduced vegetation, and human heat generation—create micro-environments that subject local microbial populations to sustained elevated temperature selection pressure. In urban soils, on building surfaces, and in the environmental niche around built structures, fungal populations chronically exposed to 1–5°C higher temperatures than historical norms may be evolving thermal adaptations through selection for existing genetic variants or through new mutations. Studies comparing thermal tolerance ranges of fungal isolates from urban and rural environments are beginning to document these differences, providing evidence for temperature-driven selection in urban environments.
Which fungal species are showing signs of thermal adaptation in cities?
Research documenting thermal adaptation in urban fungal populations is at an early stage, with most evidence from comparative studies rather than long-term evolutionary monitoring. Cryptococcus species isolated from urban environments have been found to grow at slightly higher maximum temperatures in some studies. Aspergillus fumigatus, which already has an unusually high thermal tolerance for an environmental fungus (able to grow up to 50°C), may be experiencing further selection at its upper growth range in urban environments. Some Candida species appear to differ in thermal optima between urban and hospital (human-associated) isolates versus environmental isolates. Establishing rigorous longitudinal monitoring programmes is a research priority to detect changes in thermal biology over time.
Do urban birds and rodents play a role in fungal thermal adaptation?
The hypothesis that urban wildlife may act as ‘thermal bridging’ hosts—animals whose chronically elevated body temperatures (combined with urban heat) create selection environments for fungi with higher thermal tolerance—is scientifically plausible but not yet empirically demonstrated. Urban pigeons, sparrows, rats, and mice have higher baseline metabolic rates in urban environments and experience chronically elevated ambient temperatures. If fungi colonising these animals are selected for higher thermal tolerance over multiple host generations, they may develop thermal biology more compatible with human infection. Cryptococcus gattii’s expansion from its traditional tropical range into temperate urban environments is sometimes discussed in this context, though the mechanism of expansion involves multiple factors beyond thermal adaptation.
What is being done to monitor this emerging risk?
Several research and surveillance initiatives are addressing urban thermal fungal adaptation as an emerging risk. The Casadevall laboratory at Johns Hopkins has proposed systematic monitoring of thermal tolerance ranges of environmental fungal isolates as a component of climate-change-related infectious disease surveillance. WHO’s 2022 Fungal Priority Pathogens list, which identified C. auris as a critical-priority pathogen, implicitly acknowledged climate-related thermal adaptation concerns. Some national public health laboratories are building urban environmental fungal monitoring capabilities. The fundamental challenge is establishing baseline data—without knowing what thermal tolerance values looked like in urban fungal populations 20–30 years ago, current values cannot be interpreted as ‘elevated.’ Establishing longitudinal monitoring with archived isolates or resampling historical study sites is a priority.
