Our bodies consist about 30 trillion human cells, but they are also home to about 39 trillion microbial cells. These teeming communities of bacteria, viruses, protozoa, and fungi in our guts, mouths, skin, and elsewhere—collectively called the human microbiome—don’t just consist of freeloaders and pathogens on the prowl. Instead, as scientists increasingly appreciate, these microbes form ecosystems essential to our health. A growing body of research aims to understand how disruptions to these delicate systems can rob us of the nutrients we need, interfere with the digestion of our food, and possibly trigger afflictions in our bodies and minds.
But we still know so little about our microbiome that we’re just beginning to answer a much more fundamental question: Where do these microbes come from? Can they spread from other people like a cold virus or a stomach virus?
Now the largest and most comprehensive analysis of human microbiome transmission has provided some important clues. Research led by genomics at the University of Trento in Italy found indications that organisms in the microbiome jump around a lot between people, especially those who spend a lot of time together. The resultspublished in January in Naturethey fill important gaps in our understanding of how people assemble their microbiomes and reshape them throughout their lives.
Other scientists have applauded the study. Jose Clemente Litranassociate professor of genetics and genomic sciences at the Icahn School of Medicine at Mount Sinai, hailed the work as “outstanding” and said it provided the first clear measure of how much sharing can be expected between family members or those who live together.
The study also fuels intriguing speculation about whether microbes may increase or decrease our risks of diseases such as diabetes or cancer, and thus add a communicable dimension to diseases not generally considered contagious. For brett finlayprofessor of microbiology at the University of British Columbia who wrote a comment for Science in 2020 on that possibility, the findings “put the last nail in the coffin that noncommunicable diseases maybe shouldn’t be called that.”
unfathomable diversity
Microbiomes are like fingerprints: so diverse that no two people can have the same ones. They’re also incredibly dynamic: They grow, shrink, and evolve so much throughout a person’s lifetime that a baby’s microbiome will look drastically different when they grow up. A handful of microbial species are found on more than 90 percent of people in Westernized societies, but most species are found on 20 to 90 percent of people. (Even Escherichia coliwhich is probably the only intestinal bacteria that most people could name, does not reach 90 percent frequency). Studies suggest that non-Westernized societies have an even greater diversity of microbes and more variable microbiomes.
Within a population, two randomly selected individuals typically have less than half of their microbiome species in common; On average, the overlap in the microbial composition of the gut is between 30 and 35 percent. Microbiologists debate whether there is a “core” set of microbial species that all healthy people have, but if there is, it is likely to be a single-digit percentage of the total.
However, determining how often microbes pass between people is a much more formidable problem than looking for species. A single species can consist of many different strains or genetic variants. Therefore, researchers need to be able to identify individual strains by looking at the genes in microbiome samples. And in a human microbiome, anywhere from 2 million to 20 million unique microbial genes can be present, and microbes are constantly rearranging their genes, mutating, and evolving.