The built-environment microbiome

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.

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.