The last thing most people would want in their bodies is mucus laden with viruses. But a new study suggests that viruses called bacteriophages, or phages, grab onto mucus and then infect and kill invasive bacteria. The finding, reported May 20 in the Proceedings of the National Academy of Sciences by Forest Rohwer of San Diego State University and colleagues, could mean that some viruses partner with animals and humans to stave off bacterial infections and control the composition of friendly microbes in the body.
Bacteriophages are viruses that break open bacteria, killing them. Researchers have studied bacteriophages for decades, and some disease therapies take advantage of the viruses’ bacteria-slaying abilities, says microbiologist Frederic Bushman of the University of Pennsylvania medical school. But the study provides what Bushman says is a revelation that should have been obvious; phage may be a natural part of the immune system. “It’s new in a way that is sort of common-sensey,” he says.
Previously, researchers thought of mucus mainly as a physical barrier to keep invading organisms from entering the body. The slimy substance made by our noses, intestines and other organs also fights invaders with antimicrobial molecules. Some researchers had found bacteriophages stuck in mucus, but they figured that the mucus had stopped or slowed the viruses. No one realized that the viruses are part of the body’s defense, says study coauthor Jeremy Barr, who works in Rohwer’s lab. “This is a natural use of phage therapy that has probably been around since mucosal surfaces evolved,” Barr says.
Rohwer, who studies corals, had noticed that phages tend to concentrate in mucus. To find out why, the researchers collected mucus from human gums, sea anemones, fish, corals and mouse intestines. Mucus layers had more phages and fewer bacteria than the surrounding environment, suggesting that the viruses helped to limit the number of bacteria allowed into the mucus.
Phages are coated in proteins that latch onto sugars called glycans, anchoring the viruses in the mucus, the team discovered. From there the phages can ambush encroaching bacteria.
So far, the researchers have demonstrated that mucus and phages can work together to protect cells in a dish. The next step, Bushman says, would be determining what happens inside an organism, an experiment the researchers are already planning.