We may not realize it, but our immediate environment is full of microbial communities. And, our houses have differences in microbes found on walls in various rooms. In this study, they found you could identify room use by the microbes on walls.
Another finding, was the more we seal off our houses with walls, windows, and insulation - the less microbial diversity on the walls (especially soil microbes). Instead, our walls tend to keep our human associated bacteria (E. coli, Steptococcus mutans, and on) in higher concentrations than found in rural, more open air houses.
So what does this mean? Well right now we just know there are differences in the microbial communities in urban versus rural houses. The next phase of research needs to look at how this impacts our personal microbiome on our skin and digestive tract. We may find that the urban environment has an impact on our digestive tract health.