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” . . . .The population of viruses varies depending on what bacteria the host animal is usually exposed to, which means that the system learns to respond to threats just like the other parts of the immune system.
Mucus mainly consists of huge molecular complexes called mucins, which are made up of thousands of glycan sugars attached to a central protein backbone. The team showed that phages stick to these sugars. The glycans are constantly changing and extremely variable, but the phages have equally diverse proteins in their coats, which allow them to cling to this inconsistent environment. The team showed that the presence of phages reduced the number of bacteria that can attach to mucus by more than 10,000 times.