Treated Wood Surfaces Harbor Fewer and Less Diverse Bacteria Than Raw Wood

Should the wooden handrail in a daycare center be treated or left raw? It sounds like an interior design question, but it is also a microbiology one. A study from the University of Helsinki suggests that the answer has real implications for which bacteria take up residence on indoor surfaces - and how well they survive there.

Two bacteria, two testing environments

The research team investigated bacterial adhesion, survival, and transmission on untreated and treated wood surfaces under both laboratory and real-world conditions. In the lab, they focused on two species: Staphylococcus epidermidis, a common and typically harmless bacterium that lives on human skin and transfers easily to surfaces through everyday contact; and Pseudomonas aeruginosa, an opportunistic pathogen known for causing infections in vulnerable populations and for its ability to survive harsh conditions.

The laboratory results were clear. More bacteria adhered to untreated wood surfaces and remained viable there compared with treated wood. The natural porosity and texture of raw wood appears to provide bacteria with more places to attach and persist.

Field experiments in public spaces reinforced this pattern. On untreated wood surfaces, researchers observed a greater number, greater diversity, and greater viability of bacteria compared with treated surfaces. Doctoral researcher Elina Kettunen noted that surface treatment affects the microbiota that forms on wood, which in turn may influence hygiene and health risks in indoor environments.

A trade-off worth considering

The findings are not entirely one-directional, though. Kettunen pointed out that untreated wood may help preserve beneficial microbes. There is growing scientific interest in the role of diverse microbial exposure in immune system development, particularly for children. The so-called hygiene hypothesis suggests that overly sterile environments may contribute to allergies and autoimmune conditions. Raw wood surfaces, by supporting more diverse microbial communities, could in principle contribute to the kind of microbial diversity that some researchers believe is beneficial.

This creates a genuine trade-off. In a hospital or food preparation area, reducing bacterial colonization is clearly desirable. In a home with young children, the calculus might be different. The study does not resolve this trade-off - it simply provides evidence that the choice of wood surface treatment has measurable microbiological consequences.

Environmental factors complicate the picture

The laboratory and field results were broadly consistent, but environmental factors introduced complexity. In the controlled lab setting, S. epidermidis survived better on untreated wood in isolation. In public spaces, where many bacterial species compete for the same surfaces and environmental conditions fluctuate, the dynamics were more complicated. Competition between microbes, temperature variation, humidity, and human contact patterns all influenced which species colonized which surfaces.

Associate Professor of Wood Material Science Tuula Jyske noted that indoor environmental health, material durability, and microbial management are all central concerns in construction. The findings could apply to settings from furniture to wall paneling, where the choice of material and treatment might influence hygiene outcomes.

Significant limitations

The study examined a limited number of wood materials and surface treatments. Only two bacterial species were tested in the laboratory, out of the thousands that colonize indoor surfaces. The field experiments captured a snapshot of microbial communities at specific locations and timepoints, but did not track changes over months or years.

Long-term effects of surface treatment on microbial communities remain unstudied. Treated surfaces may lose their antimicrobial properties as coatings wear down over time, potentially narrowing the gap with untreated wood. The researchers acknowledge that further work is needed to establish durable, long-term effects and to test a broader range of materials, treatments, and bacterial species.

The research also did not measure health outcomes. Demonstrating that treated wood supports fewer bacteria is different from demonstrating that treated wood reduces infection rates or improves health. That connection would require epidemiological studies linking surface treatment choices to actual health outcomes in building occupants.

What the study does establish is a starting point: the surfaces we choose in our buildings are not microbiologically neutral, and the decision to treat or leave raw has consequences that extend beyond aesthetics.