If you think your home is a haven of sterility then read no further. Microbiologist Noah Fierer has taken his microscope to our secret indoor ecosystems and tells Graham Lawton why he feels like a 19th-century natural historian
What made you decide to study indoor environments?
There has been a lot of work on insects that live in the home - bed bugs, fleas, mites and so forth. People have also been interested for a long time in the harmful microbes that live in buildings: where in the kitchen do we find salmonella, for instance? We now know that pathogens are a minuscule fraction of all the bugs in our homes. Only in the past few years have we been able to describe the full extent of diversity that lives in the home, due in part to methodological advances.
One study you did was in a public bathroom. Could you talk me through that?
We picked a restroom on the university campus here. Some of the results were confirmatory. For example, we saw more faecal-associated bacteria on the toilet seat, a lot of skin-associated bacteria on places you touch with your hands and more soil-derived bacteria on the floor. None of that was shocking, but what was interesting was we found subtle patterns that we didn't think we'd be able to detect.
Tell me about the patterns of bacteria you saw. We could differentiate male and female restrooms by identifying vaginal-associated bacteria.
It's kind of like 19th-century natural history. We're going in and saying, what's here, what's the diversity of microbes on these surfaces and how are they arranged in space? Instead of making a journey up the Amazon we're walking down the hall to the bathroom. Not nearly as glamorous, but that study was just the start.
If that was just the start, where are you planning to go with this work?
Our latest study looked at the kitchen in much more detail than ever before, and produced a three-dimensional map of microbes on every possible surface throughout the room. It sounds kind of trivial, but it's like the map that was made of plants in north America. We've had such maps since the 19th century but we don't have equivalent maps of microbes. And though we spend a large amount of time in kitchens and our homes in general, we really don't have a good understanding of how bugs are distributed, even though they may directly or indirectly influence our health and well-being.
So how many bacterial species are there in, say, a typical bathroom, and did you find anything rare or previously undiscovered?
There are hundreds of different species. A lot of them we know quite a bit about. Others are bacteria that are difficult to grow in the lab, so we don't yet know much about them. That's true whatever environment we look at - soil, the deep sea and definitely the bathroom.
Are these bacteria that have been temporarily deposited or are we talking about permanent communities?
I think the answer is both. For example there are a lot of bacteria from skin that make it onto surfaces. They're probably not growing, they are just left behind. We are constantly shedding bacteria. But some of them probably are living there, such as those in the refrigerator or on the cutting board.
So what is growing where in the kitchen, on a typical chopping board, for example?
We get a lot of bacteria that likely came from vegetables. That's not surprising. But then we also see others that were probably selected for life on the cutting board. In other words, they probably do pretty well even though the board is constantly being washed. We do pick up some bacteria that are related to known pathogens but I don't think people should be paranoid. The fact is, you're not living in a sterile environment no matter how often you clean - these bacteria are all over the place and they're nothing to be afraid of.
Does each home have its own microbial signature?
That's what we're studying now. We're sampling 1000 homes across the US to get at questions like, how different are individual homes? Does it matter if you live in a humid environment versus an arid one? If you have dogs in the house, does that affect the types of microbe in your home, or if you're vegetarian? Are there different microbes if you live in a forest or a city? These are big, basic questions that we don't yet know the answers to.
Do you have any understanding of how the ecosystems function? Are there food webs?
There's undoubtedly a food web, though we don't know much about it yet. There are fungi and bacteria that feed the mites and so forth, and it's interesting to think of our homes as mini-ecosystems, as places where microbes move around, reproduce and are consumed.
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