Modern science has revealed a startling fact that was first intimated by Anton von Leeuwenhoek scraping his teeth more than 400 years ago—you are more bacteria than you. Estimates put the number of microbial cells as constituting 10 times more of the cells in your body than actual human cells. What's worse, you better not get rid of them. Without them, you'll die.

This human microbiome, as scientists like to call it, has been the subject of much recent scrutiny, including a project to catalog the thousands or even millions of microbes within us and their function. After all, a better understanding of microbes and their role in our bodies could be the key to multiple health improvements and have led to new techniques, such as fecal transplants. Now a team of scientists led by microbiologist Jeffrey Gordon of Washington University in St. Louis has sequenced the "virome" or genetic code of "virus-like particles" from the poop of four pairs of female twins and their mothers, pulled from the Missouri Adolescent Female Twin Study that enlisted such women born between 1975 to 1986.

Perhaps unsurprisingly, female twins and their mothers tend to share the same set of such bacterial genetic material in their guts. After all, mothers inoculate infants with their gut bacteria during normal vaginal birth, and even C-section babies are almost instantly colonized by bacteria more commonly found on skin. But they don't share the same set of viruses—even though those viruses are living in roughly the same set of microbial cells. "Viromes are unique to individuals regardless of their degree of genetic relatedness," the researchers write in the July 15 issue of Nature. (Scientific American is part of the Nature Publishing Group.)

Such diversity could help explain the different health outcomes observed in even the most genetically related people, such as twins. And the virome is a treasure trove of new genetic material—more than 81 percent of the viruses or virus-like particles were new to science. This inner voyage of tiny discovery (with outsized potential) is just getting underway.

Image: © Berkebile