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Column: Our Tiniest Roommates – The Microbiome of the Built Environment

In the fifth installment of his column, ‘Just 10% Human,’ Daniel Sprockett looks at the relationships between our microbiome and that of rooms, buildings, objects within buildings, pets within buildings, and environments surrounding buildings. 


Sydney has been wet this winter. Very wet. We saw 316 mm of rainfall in June, more than double the monthly average. The last eight days of the month alone had nearly 240 mm of precipitation. With things so wet outside, my wife and I have been spending a lot of time in Sydney’s fantastic museums, theatres, and shopping districts, and almost no time outdoors.

The Australian government’s Department of Sustainability, Environment, Water, Population, and Communities reports that the average Australian only spends around 10% of their time outdoors, which is on par with the rest of the developed world. However, 10% still translates to about 2.5 hours a day, and I know I haven’t been spending even that much time outside recently. Other estimates are closer to 2-4%, or 30-60 minutes per day.

The remaining 96-98% of lives is spent indoors. We breathe air that is filtered through mechanical ventilation systems, and then heated by furnaces, or cooled by air conditioners. We walk on carpets made from synthetic fibers, or plastic resin floors held in place by industrial epoxies. Most of the surfaces we touch in our homes and buildings are hard, dry, and disinfected regularly with soaps and detergents. Human life on the inside is vastly different than life on the outside.

Microbial life is quite different inside, as well.

I’ve been discussing some of the various ways that the 100 trillion microbes living on and in your body impact your health and behaviour, but it is equally important to understand which microbes we come in contact with, and where they come from. These and similar questions are driving microbiologists and ecologists to collaborate with engineers and architects to investigate the microbiome of the built environment.

The built environment includes not just the wood, steel, and other physical materials that make up our homes, but also the greater environments that surround buildings and the people/objects that occupy the space inside. The microbes, plants, and animals found in these three components of the built environment interact with each other to form a complex, dynamic network that can be studied like any other ecosystem.

When a person first enters a room that they’ve never been in before, their body and the room normally harbor distinct microbial communities. Overtime, the two ecosystems interact, and several things could happen.

The first option is that nothing happens. The person and room microbiome might stay completely distinct. An ecosystem that doesn’t change much in the face of perturbation is said to be highly resilient, and ecological resilience is generally thought of as being a good thing because it guards against foreign invaders. One major exception to this rule is when the state of the ecosystem is causing an illness, and then you want it to change.

More commonly though, either the human microbiome is invaded by microbes already in the room, or conversely, the microbes on their body begin colonizing the room’s air and surfaces. Only a subset of microbes that live on your hands can also live on your countertop, and similarly only a subset of the microbes that live in your carpet can also live on you feet. Generally, one microbial ecosystem isn’t able to completely overtake another, but interactions between microbes can cause ecosystems to rapidly alter states. The speed at which an ecosystem changes depends on many factors, but we know that low levels of species diversity tend to predict low levels of resilience, resulting in rapid changes in response to perturbations.

Because of this, microbes tend to flow from our relatively high-diversity bodies onto the low-diversity surfaces we come in contact with. We are constantly shedding a cloud of dust, skin cells, and microbes everywhere we go, leaving behind around 1.5 million skin cells and over 15 million bacteria every hour. Normal, healthy humans leave a microbial fingerprint on surfaces that we touch, and unsurprisingly, the similarity between our homes and our bodies depends in part on how often we clean.

But cleanliness isn’t the whole story. Researchers have also found that while most of the microbes in our homes come from our bodies, factors like temperature and humidity, as well as the surrounding outside environment, help determine the structure of the microbial communities that are found on our floors, furniture, and in our air.

Interestingly, several studies have also found that the presence of pets has an enormous influence on the amount and types of microbes that live in your homes. How surprising this is to you probably depends on if you are a pet owner. I grew up with dogs, so I know firsthand that the amount of slobber and dirt they contribute to the household can be substantial. What I did find surprising was that pets also greatly impact the microbes found on your body. One recent study looked at couples that had pets, children, both, or neither. They found that cohabitating couples had more similar skin microbes than people that don’t live together, but also that the presence of a dog made couples share more microbes than couples without dogs. Pets, it seems, can act as a microbial conduit for couples. Some adults shared more microbes with their dogs than they do with their own children!

But just as we alter the microbial communities of the building we inhabit, these environments can also greatly influence our microbiome, and therefor our health. Two major projects, The Home Microbiome Study and The Wildlife of Our Homes, have begun sampling hundreds of homes and their inhabitants in an attempt to systemically characterize the diversity of bacteria, fungi, protists, and insects that live in our homes. Another massive venture, The Hospital Microbiome Project, is taking a more direct look at the interactions between microbiology, architecture, design, and human health and wellbeing.

Researchers in the United States have begun sampling a brand new hospital in Chicago as it was being built, and have continued to take daily and weekly samples of air filters, sinks, showers, floors, desktops, as well as patients and medical staff. Altogether, investigators will collect well over 12,000 samples over the course of a year. In addition to microbial samples, they are also collecting data on airflow, humidity, temperature, and building materials, as well as patient information such as length of stay and health outcome.

These data will allow investigators to test hypotheses and make predictions about how patient health and length of stay influence the microbes in their hospital rooms, and how transmission of healthcare-acquired infections might be happening via contaminated surfaces and airflow. Their goal is collect enough information that they can make robust conclusions about where microbes persist in hospital environments, how they got there, and which types of interventions (i.e. surface disinfectants vs. hand washing, forced air vs. passive ventilation, etc.) are most effective at preventing the spread of infectious disease. The project is ongoing, but all of their protocols, data, and conclusions will be made freely available on their website (hospitalmicrobiome.com), in addition to being published in academic journals.

Previous research into the hospital microbiome revealed several striking patterns. When investigators compared the microbial communities from rooms with standard HVAC ventilation to rooms with passive ventilation from open windows, they found that the rooms with open windows had much higher microbial diversity. However, the proportion of likely human pathogens was much higher in the rooms with mechanical ventilation, suggesting that some infectious agents may be transmitted through forced air ventilation systems. Simple interventions like opening a window could help reduce the number of annual healthcare-acquired infections, which is the most common complication in Australian hospitals. You can watch lead investigator Jessica Green describe this study in more detail during her TED talk from 2011 (also embedded below).

The Great Indoors” is one of the next frontiers for ecologists wanting to better understand the ecosystems we spend the majority of our lives in contact with. We both shape and are shaped by our environments, and making design and building management decisions informed by the study of microbiology can result in healthier, more productive buildings and lives.


Daniel Sprockett is a researcher at the Case Western Reserve University School of Medicine in Cleveland, Ohio. He currently resides in Double Bay with his wife, Andrea, while she completes a Master’s of International Public Health at the University of Sydney. Dan will return to the United States in September, when he begins his PhD in Microbiology and Immunology at Stanford University.