Animals use mucus as a protective barrier in places like the gut and lungs, and that mucus is full of bacteria-eating 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.
Microbial ecologist Forest Rohwer found that mucus contained four times more phages than the surrounding environment:
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, which Barr likens to a “large biological bottlebrush”.
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.
Barr says he thinks that the phage strains found most often in mucus will be those that target the most common bacteria, providing a sort of ‘mucus memory’ against the most relevant local microbes. But because mucus is continuously being shed and replenished, these relationships are in constant flux. Barr, Rohwer and the team are now trying to simulate the evolutionary dynamics within this realm of mucus.