# The built-environment microbiome

Why the built-environment microbiome behaves like an ecosystem: human shedding and ventilation as source and dispersal, dry surfaces as sinks where DNA outlives cells, why a sterile surface is an empty niche, and how probiotic seeding, materials and humidity act on the same ecology.

Indoor microbial communities are an ecology with sources, sinks and dispersal routes; sterilization ambition collides with that ecology, and the collision usually selects for the organisms you want least.

Source: https://en.bioecon.ru/docs/ecology-restoration/monitoring-conservation/urban-microbiome-management/
Updated: 2026-09-07



A building is not cleaner than outdoors; it is differently populated. Its microbial community arrives from identifiable sources, moves along identifiable routes, and answers design choices — ventilation, materials, moisture, cleaning regime — the way any ecosystem answers its environment. Managing that community honestly means managing an ecology, and the central failure mode is believing it can be deleted.

## Where indoor microbes come from

The dominant source indoors is people: skin and oral organisms shed continuously into air and onto surfaces, so a crowded room is measurably richer in human-associated taxa than an empty one. Secondary sources are outdoor air and soil infiltrating from outside, plumbing and drains, and damp materials growing whatever arrived as spores. Dispersal is engineered: ventilation either dilutes and removes or recirculates and homogenises, and filters decide what survives the trip. Surfaces, meanwhile, are mostly sinks rather than habitats — indoors they are dry, nutrient-poor and hostile, so most organisms detected there are dead or dormant. That is also a measurement trap: metagenomics reads DNA, and DNA outlives the cell, so "detected" rarely means "viable and growing" — the same trap as in [aquatic eDNA monitoring](../edna-invasive-species-biomonitoring/).

## Why the sterilization ambition collides

A sterile surface is an empty niche, and niches fill. Recolonisation is by whoever arrives first, and the arrival set is the building itself: drains, shoes, hands, air. Worse, broad-spectrum biocides are a selection pressure applied at building scale: resistant survivors repopulate the treated surface, resistance mechanisms tend to be broad, and the genes that shrug off a surface disinfectant often carry cross-resistance to clinical antibiotics. The result of a sterilise-and-hope regime is a community no smaller, only worse composed. That is the logic behind probiotic cleaning — seeding cleaned surfaces with harmless colonists that occupy the niche competitively before other arrivals can — and behind the building-science interest in materials and humidity regimes that support a diverse, benign community instead of fighting for zero. Where hygiene is real it is untouched: hands in clinical settings and food-contact surfaces are handled with targeted, evidence-backed protocols.

## What is genuinely hard

Composition is easy to measure; health is not. Linking a community profile causally, rather than correlationally, to occupant outcomes requires intervention studies that buildings rarely host. The viable-versus-detected gap distorts risk assessment, sampling covers a vanishing fraction of the surface a building exposes, and community dynamics are context-specific enough that results from one climate and ventilation regime transfer poorly to another. The realistic goal the field has converged on is not the germ-free building but the well-composed one: dilution and filtration in the air, dryness and material choice on surfaces, and interventions judged by the community they leave behind, not by the momentary count they produce.