Greg Caporaso was sifting through blog posts on microbe.net, which covers the microbiology of built environments, when a study idea sparked for him and colleagues Jeff Siegel, Scott Kelley and Rob Knight.
“It became clear to me that there was a lot of interesting work being done to understand how bacteria or fungi might impact the health of building inhabitants, and how very little was known about what microbes live in the ground, our offices, our cars, hospitals and homes,” says Caporaso, an assistant professor of biological sciences and assistant director of the Center for Microbial Genetics and Genomics at Northern Arizona University in Flagstaff.
He noticed that there hadn’t been good systematic investigations into how different surfaces might drive compositions of microbial communities in the built environment. His most recent study, published this week in mSystems, offers insight.
While previous studies have suggested that individual offices have their own microbial communities, his study found something else: Cities have their own distinct microbial communities but these communities don’t vary much between offices located in the same city.
Sampling microbes from nine offices in three North American cities, Caporaso and colleagues also found that human skin contributes heavily to the composition of built environment surfaces and that office floors have more microbes than other surfaces, likely because of soil and other materials deposited from workers’ shoes.
“What we think these previous studies are detecting is the personalized human microbiome effect,” Caporaso says. “When you touch your desk or your phone, you leave behind some microbes from your skin.” Previous studies may have sampled a desk surface or doorknob but his study placed notes by the sample plates asking office workers not to touch or contaminate the materials.
“We suspect that in the absence of extreme conditions like flooding, microbes may be passively accumulating on surfaces in the built environment rather than undergoing an active process,” he says. “As we continue to expand our understanding of the microbiology of the built environment, possibly including routine monitoring of microbial communities to track changes that may impact human health, our results will help inform future research efforts.”
The researchers monitored three offices over a one-year period in each of the following cities: Flagstaff, San Diego and Toronto. In each office they installed three sampling plates, with one plate each on the floor, ceiling and wall; each plate contained two or three swatches each of painted drywall, ceiling tile and carpet, as well as sensors that allowed investigators to monitor parameters of the environment including equilibrium relative humidity on the surfaces of the swatches, available light, occupancy, and temperature. Samples were collected in four six-week sampling periods, one per season. Then, they used 16S rRNA gene sequencing and ITS-1 to profile the samples’ bacterial and fungal communities. [Image: Researcher John Chase samples office surface materials just prior to installation in June, 2013. (Photo credit: Greg Caporaso)]
The team found that floor samples, regardless of material, contained more microbes than wall or ceiling surfaces; that frequent sampling of the test plates disrupted the microbial communities only slightly; and that cities had their own signature microbial communities.
“This was especially interesting because even within each city, the offices we studied differed from each other in terms of size, usage patterns, and ventilation systems,” says Caporaso, “suggesting that geography is more important than any of these features in driving the bacterial community composition of the offices within the ranges that we studied.”
The Flagstaff offices had richer microbial communities than those in San Diego or Toronto, which were more similar to each other, though Caporaso says it’s unclear why.
To see if any particular office workers or body sites were sources for the microbes seen in offices, the researchers also collected human skin, nasal, oral, and fecal microbiome samples from 11 workers at one of the Flagstaff offices, and from individuals performing the sampling techniques in all three cities.
Across all nine offices, the team found that human skin bacterial communities were the largest identifiable source of the office bacterial community samples, with at least 25-30 percent of the office surface microbiome derived from human skin. The human nasal microbiome also appeared to be a small but consistent source of office surface microbial communities. The largest source of microbial communities in these offices, however, was from non-human sources such as the environment. Researchers found no significant associations, however, between the office microbes collected and indoor environmental variables such as temperature or humidity.
The team will next simulate flooding events to examine how fungal communities in the built environment change over time.