Mapping our microbial footprints

Despite how hard we may try to scrub ourselves clean with antibacterial soap, it appears that humans are more tightly bound to our surrounding microbes than we thought.

Researchers from the US Department of Energy’s Argonne National Laboratory and the University of Chicago have shed light on the complicated interaction between humans and the microbes around us. The study was led by Argonne microbiologist Jack Gilbert and published in the journal Science.

Mounting evidence suggests that the microbes that live on and around us play a role in human health and disease treatment and transmission. As noted by Gilbert, “We know that certain bacteria can make it easier for mice to put on weight, for example, and that others influence brain development in young mice.

“We want to know where these bacteria come from and, as people spend more and more time indoors, we wanted to map out the microbes that live in our homes and the likelihood that they will settle on us.”

The Home Microbiome Project followed seven families, comprising 18 people, three dogs and one cat, over the course of six weeks. The participants in the study swabbed their hands, feet and noses daily to collect a sample of the microbial populations living in and on them. They also sampled surfaces in the house, including doorknobs, light switches, floors and countertops.

Illustration by Renee Carlson/Argonne National Laboratory. Shared under CC BY-NC-SA 2.0.

The samples came to Argonne, where researchers performed DNA analysis to characterise the different species of microbes in each sample. They found that people substantially affected the microbial communities in a house.

“Microbial communities differed substantially among homes, and the home microbiome was largely sourced from humans,” the researchers said. When three of the families moved house, it took less than a day for the microbial make-up of the new house to look just like the old one.

Couples were found to share more microbes with one another than occupants who weren’t in a relationship and, if they had young children, the microbial community would be shared with them as well. Hands were most likely to have similar microbes, while noses showed more individual variation.

In at least one case, the researchers tracked a potentially pathogenic strain of bacteria called Enterobacter, which first appeared on one person’s hands, then the kitchen counter, and then another person’s hands.

“This doesn’t mean that the countertop was definitely the mode of transmission between the two humans, but it’s certainly a smoking gun,” Gilbert said.

“It’s also quite possible that we are routinely exposed to harmful bacteria - living on us and in our environment - but it only causes disease when our immune systems are otherwise disrupted.”

Gilbert believes home microbiome studies could potentially serve as a forensic tool. Given an unidentified sample from a floor in this study, “we could easily predict which family it came from”, he said.

The research also suggests that when a person (and their microbes) leaves a house, the microbial community shifts noticeably in a matter of days. Therefore, said Gilbert, “You could theoretically predict whether a person has lived in this location, and how recently, with very good accuracy.”

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