In recent years, the human microbiome has gotten a lot of press. We have read about how our gut microbes affect our eating habits, immune system and mind. But what about the microbes that surround us indoors? We spend most of our time indoors, at home, in our cars, in offices and other buildings. We interact with these “built environments” (BE) extensively, leaving microbes from our skin on the surfaces we touch and also acquiring microbes that are already present on these surfaces. Airborne microbes from the outdoors or from humans also accumulate on these surfaces and travel throughout our indoor spaces. In a recent paper from the Caporaso lab, the individual factors that affect the microbiome of BEs were explored.
Built microbiome study design
The goal of their research was to answer questions about how distinct factors affect urban microbiomes. How do seasons affect BE microbiomes? Do surface material influence the composition of microbial communities on these surfaces? How do the personal microbiomes of individuals that use these BEs affect the surface microbiome?
These researchers surveyed the microbiome of nine offices: three in Flagstaff, AZ, three in San Diego, CA and three in Toronto, ON. Sampling plates were placed on the floor, ceiling and wall at each location. Each plate consisted of swatches of painted drywall, ceiling tile and carpet. Sensors were also included to monitor temperature and humidity. Swatches were sampled in four sampling periods (one per season) and bacterial and fungal species were determined by sequencing bacterial and fungal genetic markers in these samples.
Cities have their own microbial signatures
The group found that the location, not the building material, drives microbial community composition. Floor samples were richer than ceiling or wall samples no matter the material. Cities also have their own signature microbial communities. In other words, offices within the same city look more like one another than those of other cities.
Microbiome of individuals have minimal effect
How does a person’s unique microbiome affect the office microbiome? The researchers collected human skin, nasal, oral and fecal samples from 11 inhabitants of the office to identify any similarities between the microbiome of the surfaces and the individuals. About 25-30% of the office surface microbiome was derived from human skin, and the human nasal microbiome was also a small source of office surface microbiome. However, the largest source of microbial communities in the office surfaces were of nonhuman origin.
It also appeared that the microbial composition on these surfaces were not influenced by the personalized microbiome of the individuals that inhabited the office. In other words, the surface microbiome of one office did not seem to better match the personalized microbiomes from that office compared to the personalized microbiome of another office. The experiment was designed such that the inhabitants were instructed not to touch the sampling plates and any microbes that were found on these surfaces were airborne. The researchers suspected that since other studies have shown that personalized microbiomes have been transfered to surfaces through direct contact, the inability to detect personalized microbiomes on the sampling plates was due to the lack of direct contact and that the personalized microbiome has far less of an effect on surfaces than we would have expected.
Passive accumulation of microbes
Lastly, the researchers concluded that microbes may be passively accumulating on surfaces rather than undergoing an active succession process where microbial interactions drive changes in community composition. The researchers found that the microbial composition on these surfaces remain relatively unchanged in their yearlong study. Thus, the bacteria seem to be in a dormant or non-replicating state that resemble a dessert environment waiting for water to become active again. Despite this result, other aspects of BEs (ex: ventilation source, airflow rates, direct human contact, etc) can create microbial communities that are capable of much more than passive accumulation.
The future of studying built environment microbiomes
The microbiome of urban environments have largely been understudied despite its implications in public health. Understanding the “microbial baseline” of a city could help public health officials identify abnormalities associated with a disease outbreak. Specifically in buildings, the relative abundance of pathogenic bacteria indoors is higher than outdoors. Thus, further research on BE microbiome can also influence building design to maximize human health as both architectural and environmental conditions may affect BE microbes.
