Despite comprising a fraction of the microorganism biomass of outdoor environments, the microbial ecology of built spaces poses significant implications for human immune, neurological, and endocrine health. In a recent review published in the journal Nature Reviews Microbiology, researchers elucidate the literature on the indoor microbiome, microbial dynamics within built spaces, and their association with human well-being.
They focus on the relationship between urbanization design choices and observed surges in antibiotic-resistant bacteria and highlight technological advances and targeted interventions that may improve population-wide public health outcomes in the future.
Study: The indoors microbiome and human health. Image Credit: Shutterstock AI
Background
The term' indoor microbiome' comprises all bacteria, fungi, protists, archaea, and viruses found within human-built environments (buildings), particularly those used for human habitation. The indoor microbiome is a dynamic ecosystem, with its communities constantly changing as a function of movement (of its human and non-human occupants) and inputs from surrounding outdoor spaces (soil, water, and air).
Research has revealed that the standing composition of indoor microbial communities is substantially impacted by the presence of non-human animals (pets), proximity to green spaces, occupant density, and building design choices (including materials and surface finishes). These microorganism communities have, in turn, been observed to alter human health outcomes significantly, contributing to immune desensitization (most prominent in younger individuals), changes in human gut microbiota, and surges in antibiotic-resistant pathogens.
In today's world of unprecedented urbanization and land use (human alterations of natural environments into built ones), understanding the interplay between indoor microbiomes and human immune health becomes increasingly imperative. This understanding may give urban planners and developers the scientific insights needed to optimize their land use and development choices, thereby contributing to a healthier tomorrow.
The indoor microbiome and its ecology
Like outdoor environments, indoor microbial communities are shaped by physiochemical factors, including temperature, substrate type, nutrient availability, and humidity/water. Unlike their outdoor counterparts, however, indoor microbial communities are subjected to extremely dry and often nutrient-deficient surroundings.
This has been observed to cause rapid and pronounced decreases in microbial biomass following exposure to built environments—studies have found that more than 75% of all sequenced bacterial genetic material from indoor environments belongs to dead or lysed cells. Unfortunately, the microbes that do survive are often hardy species such as Bacillus or those that thrive in plumbing pipes and other areas that are rarely accessed (biofilm-forming species such as Denitratisoma), which may have profound and sometimes adverse implications for human health.
Knowledge of the density and community composition of indoor microbiomes is essential in healthcare and educational settings. Despite frequent sanitization resulting in low microbial population densities and species richness, these settings are often dominated by pathogenic and even antimicrobial-resistant taxa, which humans may then transmit to their habitations.
"…surveillance of the built environment is crucial to understand the origins of pathogens and antimicrobial resistance and how antimicrobial-resistant infection spreads."
Health implications
The impact of the indoor microbiome on health is multifactorial. Conventional research suggests that the indoor microbiome may directly impact human health, including immunity development, infectious disease, allergy implications, and even metabolic diseases (through its influence on the human gut microbiome). Exposure, or lack thereof, to specific microbial or environmental antigens (e.g., pollen) may substantially impact individuals' immunity, particularly during their childhood.
More recent work has revealed subtle health impacts of rapid urbanization, including mental health implications and the recently described "sick building syndrome." The sick building syndrome occurs when individuals who spend prolonged periods indoors experience acute health—or comfort-related symptoms without diagnosable underlying causes. Researchers hypothesize that this condition may arise from poor ventilation, poor lighting (causing eye strain), excessive electromagnetic radiation (from electronics), and even building material choices.
Current advances in research and future perspectives
Indoor microbiome research presents the confluence of medicine, environmental science, public healthcare, and building design. Recent advancements in DNA and RNA extraction and sequencing methodologies have allowed researchers to characterize and quantify indoor microbial assemblages, thereby providing essential data for epidemiological investigations.
Future-focused research aims to realize the concept of 'healthy buildings' – design innovations tailored to achieve human well-being and environmental sustainability. Developers and designers are exploring ways to integrate artificial intelligence (AI) advances and smart devices to attain real-time air and surface quality monitoring and automated responses. A current challenge in this goal is the substantial financial burden associated with the initial building and setup of these intelligent buildings, with costs estimated to be multiple of conventional development pursuits.
"…the retrofitting of existing structures to conform to these advanced standards presents logistical and financial hurdles. Occupant acclimatization to these technologically enriched environments also raises concerns about cultural and behavioral adaptability. Moreover, the absence of uniform regulatory frameworks and standardization in defining what constitutes a healthy building adds a layer of complexity."
Conclusions
Indoor microbiomes are intrinsically linked to the health of humans living under the same roof. Advances in '-omics' techniques and the interdisciplinary nature of microbiome research are expanding our understanding of these often subtle relationships between humans and microbes and helping shape future developmental and land use choices, which may culminate in a healthier and more environmentally sustainable future urban landscape.
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