[www.nature.com]

“Mucus is everywhere,” says microbiologist Jeremy Barr. Almost every animal uses it to make a barrier that protects tissues that are exposed to the environment, such as the gut or lungs. Now, Barr and a team of researchers have discovered that mucus is also the key to an ancient partnership between animals and viruses.

Barr and his colleagues, who are based at San Diego State University in California, show that animal mucus — whether from humans, fish or corals — is loaded with bacteria-killing viruses called phages. These protect their hosts from infection by destroying incoming bacteria. In return, the phages are exposed to a steady torrent of microbes in which to reproduce. “It’s a unique form of symbiosis, between animals and viruses,” says Rotem Sorek, a microbial geneticist at the Weizmann Institute of Science in Rehovot, Israel, who was not involved in the research.

“It’s groundbreaking,” adds Frederic Bushman, a microbiologist from the University of Pennsylvania in Philadelphia. “The idea that phage can be viewed as part of the innate immune system is original and exciting.” The results are published in the Proceedings of the National Academy of Sciences1.