Bacteria and other microbes are known to be fond of sharing their genes around, transferring them to others in a process that dramatically turbocharges evolution. They do this more inside the human body than in other environments, and may even use our blood as a postal service to send genes between distant locations.
The process where DNA moves from one living thing to another without the recipient being the originator's descendant is known as “horizontal gene transfer” (HGT). Although it can happen with larger organisms – ferns are particularly partial to it, it is most common in microorganisms.
"Horizontal gene transfer is a major force of exchange of genetic information on Earth," said Professor Gustavo Caetano-Anollés of the University of Illinois in a statement. "These exchanges allow microorganisms to adapt and thrive, but they are likely also important for human health."
"Most current methods for determining horizontal gene transfer compare DNA features or statistical similarity between genomes to identify foreign genes," said Arshan Nasir of COMSATS University Islamabad, co-author of a paper in Scientific Reports revealing the findings. "This works fairly well for relatively recent gene transfers, but often fails to identify transfer events that occurred millions or billions of years ago."
Consequently, Caetano-Anollés and Nasir adopted much more time-consuming analysis and reconstructed 80,000 family trees of inhabitants of our guts, mouths, lungs, blood, urinary tracts, and skin, each of which usually has distinct microbiota. The study involved beneficial or neutral bacteria, ignoring those that cause infections. Numerous genes were tested against these to see which ones were shared between bacteria with very different ancestries.
HGT is common, accounting for the majority of genes in human microbiota, even though the analysis left out horizontal transfer between closely related species. Caetano-Anollés said; "The horizontal exchange between microbes in our bodies is about 30 percent higher than what you'll find on the rest of the planet. This implies that our bodies provide a niche that is unique and facilitates innovation at the microbe level." Surprising as this may appear, previous less detailed studies had found something similar. How and why our bodies act like innovation incubators requires further research.
Even more amazingly, bacteria are not just swapping with their neighbors. Three-fifths of the HGT took place between related bacterial species that inhabit different parts of the body, for example, the mouth and urogenital tract. Before you jump to conclusions about how that particular transfer occurs, note Nasir's caution; “Some of these could be very old gene transfer events that happened before the microbes colonized the body.”
Bacterial movement between body sites as we age also presumably contributes. However, the authors also refer to the “tantalizing possibility” that bacterial DNA is being transferred through the blood from one part of the body to another, even without the bacteria themselves.