Over the past few decades, our understanding of health has undergone a paradigm shift: the “Hygiene Hypothesis” has evolved into the “Biodiversity Hypothesis”. It’s no longer just about being “too clean,” but about our dangerous separation from the trillions of microbes in soil and plants that train our immune systems. This gap became clear during a pediatric consultation when a parent asked: 'We’ve started our "five muddy minutes" a day, but what else can we do to support our child's microbiome?'. The silence from the medical community revealed the gap: we have ignored the ecosystem that lives within us. This is a story that is very personal (what is happening inside our bodies) and globally significant (how are we changing the planet’s biology). This article synthesizes evolutionary biology, microbial ecology, landscape architecture, and socio-economic policy to provide a roadmap for the intentional design of urban spaces that prioritize microbial wealth.
The Evolutionary Context: Humans as Ecological Units
To understand why cities make us sick, we must redefine what it means to be human. We are holobionts: a host plus the trillions of microbial organisms working symbiotically to form a functioning ecological unit. Think of the 'host' as the structural framework, much like a planet or a forest, and the 'ecological unit' as the entire community of inhabitants that keep that environment alive. Just as a forest does not consist of just the trees, but also of the fungi, birds, and soil bacteria that allow it to breathe and grow. A human being is a functional ecological unit where we cannot survive without our microbial partners. With roughly 8% of the human genome being of microbial origin, we are literally part-microbe. Our biology is so deeply intertwined with these organisms that we cannot achieve full physiological or psychological function in isolation.
The “Old Friends” Hypothesis
The relationship between humans and microbial community is further explained by the “Old Friends” hypothesis, which serves as a modern refinement of the 1989 “Hygiene Hypothesis”. This theory emphasizes the role of exposures to microorganisms with which humans co-evolved as essential drivers of the regulatory and anti-inflammatory arm of the immune system. These "Old Friends" include saprophytic mycobacteria, lactobacilli, and certain helminths that were omnipresent throughout mammalian evolution. These organisms act as essential instructors for the immune system, driving both specific and bystander immunoregulation.
The Aerobiome: Our Invisible Instruction Manual
How do we actually connect with these “Old Friends”? The connection between a diverse landscape and a healthy immune system is rooted in a direct physical exchange. We are constantly immersed in an aerobiome: an invisible microbial fog of spores, bacteria, and fungal fragments shed by soil and plants. When we breath, these microbial particles land on our airways. This is where the transition from environment to immunity occurs. Specialized sensing cells in the mucosal tissue detect the unique molecular patterns of these microbes. This interaction sends a biochemical signal to the lymphatic system, triggering the production of Regulatory T cells (Tregs).
These Tregs are the immune system’s essential "peacekeepers". Tregs produce anti-inflammatory signals that prevent our systems from sliding into a state of high alert. In a biodiverse environment, this dialogue is rich and constant. In sterile cities, this conversation falls silent. Without these microbial instructors, the immune system comes a Th2-type: a hyper-reactive state that leads to the modern explosion of asthma, allergies, and chronic inflammation.
The Amish vs Hutterites
The power of this mechanism is proven by the Amish and Hutterite communities. Despite similar genetics, their health outcomes are opposites. The Hutterites use mechanized, sterile farming, while the Amish maintain traditional, animal-rich methods. Amish children, constantly exposed to a rich "microbial fog," remain healthy. In contrast, Hutterite children, isolated by industrialization, suffer from high rates of asthma. This stark contrast validates the "Old Friends" hypothesis, proving that the human holobiont cannot function in isolation. Our internal health is fundamentally dependent on the microbial diversity of the world around us.
The Problem: Sterile Urbanism & Health Inequity
Microbial diversity is a matter of social justice. While wealthy neighborhoods enjoy the "microbial wealth" of private gardens, impoverished "grey spaces" suffer from biological depletion. Research indicates that microbial richness decreases significantly as one moves away from the soil, meaning residents of high-rise social housing are effectively quarantined from the most diverse layers of the aerobiome. This isolation leads to dysbiosis: a state where the microbial community inside us becomes unbalanced or depleted. When citizens are born into these "microbial deserts," their immune systems remain "uneducated," resulting in higher rates of asthma and chronic inflammation. This creates a cycle where environmental poverty leads to physiological poverty. By reintroducing "Old Friends" into these landscapes through rewilding, we can use biodiversity to re-educate the immune system, providing a biological foundation for true health equity.
The Solution: Microbiome-Inspired Green Infrastructure (MIGI)
To bridge the gap between human and habitat, researchers Robinson, Mills, and Breed proposed Microbiome-Inspired Green Infrastructure (MIGI). MIGI is a type of infrastructure based on the “Biodiversity Hypothesis” (the bridge between the microbes and the landscape). It treats the city as a living extension of the human holobiont, intentionally engineered to maximize health-promoting interactions between environmental microbiomes and citizens.
To build a healthy city, we must design for microbial transport. Since these "Old Friends" rely on physical movement to reach us, urban planning must shift from aesthetics to the mechanics of exposure. Cities should be designed as 'permeable lungs,' where wind corridors are intentionally aligned with biodiverse parks to push the microbial fog deep into high-density residential zones. This approach, known as Probiotic Urban Design, moves beyond passive greening (adding isolated, decorative plants that lack ecological complexity for the microbiome) toward active, microbial engineering. It focuses on three primary interventions: reclaiming the biological desert of the lawn, inoculating the environments of our children, and leveraging vertical structures as microbial emitters.
Reclaiming the Biological Desert of the Lawn
The turf grass lawn is the default landscape of the Western city, yet it is a "biological desert" maintained through the suppression of microbial life. We must replace these sterile monocultures with native plant reservoirs. Research shows that native gardens support significantly higher bacterial biodiversity, including keystone taxa like Gemmatimonas and Acidobacteria. By planting species like Monarda fistulosa and Baptisia australis, we create a "rhizosphere" (the living soil zone surrounding roots) that serves as a launchpad for the very "Old Friends". For low-income residents in "microbial deserts," replacing sterile lawns with these reservoirs is a matter of health equity. It transforms a private aesthetic choice into a vital public health intervention ensuring that the "microbial fog" is available to everyone, regardless of their zip code.
Inoculating the Environments of our Children
We now have the empirical proof that ‘playing in the dirt’ is a medical necessity. The 28-day experiment in Finland remains the gold standard for this shift. By simply replacing gravel playgrounds with forest floor researchers fundamentally altered the biology of the children involved in the experiment. Within four weeks, their blood showed a surge in anti-inflammatory cytokines and "peacekeeping" Tregs. This was not a subtle change; it was a rapid biological upgrade. We must treat soil not as a pollutant, but as a living vaccine that can be scaled into every school and park.
Leveraging Vertical Structures as Microbial Emitters
In dense urban canyons where ground space is scarce, we must look upward. Green walls and living pillars act as functional lungs, circulating the "microbial fog" into the breathing zones of residents. This is the frontier of bio-integrated communication, where buildings utilize living materials like fungi-based bricks or self-healing concrete. These living substrates allow beneficial microbes to colonize the built environment, turning our buildings into active participants in the human holobiont.
Probiotic Urban Design applies the Biodiversity Hypothesis as a functional blueprint. By transforming sterile surfaces into active microbial sources, we can reverse the crisis of immune depletion. It is time to restore the urban landscape as a vital, living extension of ourselves.
Societal Catalysts: Reclaiming the Soil
The shift toward the “Biodiverse City” is not merely a top-down architectural challenge but a bottom-up social movement. Reclaiming our microbial heritage requires us to trade our fear of "dirt" for a collective stewardship of the soil.
Five Muddy Minutes: Nature as Medicine
The "Five Muddy Minutes" initiative, led by public health experts at the University of Bradford, encourages citizens to embrace contact with soil and trees. This campaign targets specific beneficial microbes which are found in soil and has been linked to reduced anxiety and improved emotional resilience.
Crucially, this starts before birth. A mother’s microbiome seeds her baby’s first defenses; therefore, access to nature for expectant mothers is vital for the next generation's health. When viewed through this lens, providing biodiverse green spaces in impoverished "grey spaces" is an essential public health intervention designed to ensure equitable health starts for the next generation. The initiative calls on schools, businesses, and community groups to ‘stop fearing dirt’ and provide opportunities for brief, daily contact with the living earth.
De-paving Movements: Reclaiming the Soil
To restore the “microbial fog”, the "De-paving" movement has emerged where community members gather to remove asphalt and replace it with pocket parks and rain gardens. In Portland, Oregon, projects like the transformation of 4,500 square feet of asphalt at Vestal Elementary into an outdoor classroom with native plants demonstrate the power of this approach. Similarly, in Ghent, Belgium, the "Rewilding Brigade" has turned the act of desealing into a social and educational activity, while residents in Manchester, UK, have reclaimed neglected alleyways as shared social-green spaces. These projects prove that urban rewilding supports both social renewal and community cohesion. Providing a sense of collective pride and ownership over the urban environment while simultaneously restoring the microbial fog.
These projects do more than fix infrastructure; they foster social cohesion and collective pride. By desealing our cities, we are not just planting trees—we are restoring the microbial heritage essential for the survival of the human holobiont.
Conclusion: The Roadmap to the Living City
To return to the question asked in the doctor’s office: "What can we do?". The answer lies in recognizing that our health is not a private matter contained within our skin, but an emergent property of the ecosystems we inhabit. The "Biodiversity City" is a vision of urban development that restores the evolutionary link between humans and the microbial world. By moving beyond the concrete desert and adopting Microbiome-Inspired Green Infrastructure, cities can become engines of public health rather than sites of chronic illness.
Achieving this vision requires a multi-faceted approach. First, it demands transdisciplinary collaboration between architects, microbial ecologists, and public health officers to ensure that urban design is truly bio informed. Second, it requires societal re-engagement through campaigns like "Five Muddy Minutes" that dismantle the cultural fear of the microbial world. Third, it necessitates physical transformation through de-paving movements that reclaim asphalt for living soil. Finally, this must be guided by equity-driven policy, ensuring that the benefits of microbial wealth are distributed to all social groups, especially those currently confined to "grey spaces." Ultimately, the rewilding of our streets is a personal and global investment. By fostering the "microbial fog", we rewild our own health, ensuring that the cities of tomorrow nurture all inhabitants, both the seen and the unseen. Shall we?
This article is part of The Outside World, ftrprf’s very own research center.
As changemakers, we believe that what happens in the outside world is the most powerful force shaping organizational strategy – and also the most underestimated. To do well, organizations need to understand what’s happening in the outside world. To do significantly better, they need to be aware of what it means for their future, their relations, their strategy, and their impact. We serve as a bridge between society and tailored strategy by analysing societal dynamics, global trends, and shifting public expectations with a multidisciplinary team of international analysts, excellent tooling, sophisticated AI, and a systems approach. This article is part of Q1 2026 research focus, which centers on biodiversity.
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