Giorgia Guglielmi in Science:
A century after they were discovered killing bacteria in the feces of World War I soldiers, the viruses known as bacteriophages, or simply phages, are drawing new attention for the role they might play within the human body. Phages have been found most everywhere, from oceans to soils. Now, a study suggests that people absorb up to 30 billion phages every day through their intestines. Though where the viruses end up is unclear, those data and other recent studies have scientists wondering whether a sea of phages within the body—a “phageome”—might influence our physiology, perhaps by regulating our immune systems. “Basic biology teaching says that phages don’t interact with eukaryotic cells,” says phage researcher Jeremy Barr of Monash University in Melbourne, Australia, who led the study published this week in mBio. He’s now convinced “that’s complete BS.” For decades, most medical research on phages focused on turning these bacterial parasites into antibiotics. There have been some compelling success stories, but phage therapy has struggled to become a dependable treatment.
Yet Barr’s earlier research showed that phages might naturally help protect us from pathogens. Studying animals ranging from corals to humans, he found that phages are more than four times as abundant in mucus layers, like the ones that protect our gums and gut, as they are in the adjacent environment. The protein shell of a phage, it turned out, can bind mucins, large secreted molecules that together with water make up mucus. This works out well for both phages and mucusmaking animals. Sticking to mucus enables the phages to encounter more of their bacterial prey. And as a result, Barr showed in a series of in vitro studies, the viruses protect the underlying cells from potential bacteria pathogens, providing an additional layer of immunity. Now, he has found evidence that these viruses can make their way from the gut’s mucus into the body. In a lab dish, his team showed that human epithelial cells such as those that line our guts, lungs, and the capillaries surrounding the brain take up phages and transport them across their interior. The transport mechanism remains unknown, but the researchers spotted the viruses enclosed in vesicles within the cells. What’s more, the cells consistently took up phages on the side that in the body faces outward, for example toward the gut lumen, and released them on the opposite, inward-facing side. From the rate at which the epithelial cells took up phages in the lab, the researchers estimated that a person might absorb up to 30 billion in a day.