After constructing miniature living rooms for microbes, a team of researchers from the University of Oregon has concluded that indoor spaces exposed to sunlight likely harbor fewer bacteria than those that are left dark. Their study, published in the journal Microbiome, did not test whether light conditions affect disease-causing species differently from harmless or mutualistic ones and only assessed bacteria that thrive in the relatively dry environment of dust, excluding those found in damp corners and crevices. However, the authors believe that follow-up work on the subject could help inform the design of safer homes, workplaces, and hospitals.

“Our results indicate that dust exposed to daylight contains smaller viable bacterial communities that more strongly resemble outdoor air communities [rather than those derived from human skin, the human gut, or soil] and that the bactericidal effects of ordinary window-filtered sunlight may be similar to those achieved by ultraviolet light wavelengths [for some bacteria types],” the authors wrote.

In order to experiment with real-world indoor bacterial communities, the authors collected dust samples from every room of seven single-family houses in the city of Eugene, Oregon. The samples were blended together, and a thin layer of the resulting mixture was spread onto petri dishes and placed into nine identical, sealable rectangular containers designed to act as mini versions of a typical living room.

The containers each had a window opening covered by one of three materials: glass that let in visible and near-infrared light but blocked most UVA and UVB radiation, like most commercial window glass; glass that blocked most visible and near-infrared but let in UVA and UVB; or an opaque aluminum plate. The internal temperatures were maintained between 18.2 and 22.3°C (64.8 to 72.1°F) and humidity was kept to between 23 and 64 percent, typical of real-world indoor conditions. The sealed microcosms were then put in south-facing building openings with no light obstructions.

After 90 days of light exposure, the number of living bacteria was significantly lower in the visible and UV light microcosms compared to the dark microcosms. The bacterial communities that had been living in either light condition were dominated by groups associated with outdoor air, whereas those in the dark had only about 25 percent outdoor air species. All three community types had low levels of skin-derived bacteria (15 to 25 percent).

The authors note that both the compositions and abundances of bacteria in UV radiation microcosms and visible light microcosms were comparable. As expected, given the diversity of habitats bacteria may thrive in, even though daylight appeared to cause a reduction in a handful of the most abundant household bacteria, a few types of rare bacteria increased during the experiment. Yet the authors assert that this may have occurred because the dominant players in the microbiome were gone, giving the bacterial bench-warmers more access to resources.