The idea that phage can be viewed as part of the innate immune system is novel and very important. Dr Jeremy Barr, a microbiologist at San Diego State University, and a team of researchers have discovered that mucus is key to an ancient partnership between animals and viruses.Mucus is nearly everywhere and almost every animal uses it to make a barrier that protects tissues that are exposed to the environment, such as the gut or lungs. Barr and colleagues show that animal mucus, whether from humans, fish or corals, is loaded with bacteria-killing viruses called phages. These phage protect their hosts from infection by destroying incoming bacteria. In return, the phages are exposed to a steady supply of microbes in which to reproduce. Thus a unique form of symbiosis, between animals and viruses, exists in our mucous.
The study’s leader, microbial ecologist Dr. Forest Rohwer, has been collecting mucus from across the animal kingdom for years, and noticed that these samples contained over four times more phages than the surrounding environment. “One of the most exciting aspects about this symbiotic relationship is that it likely acts at all mucus surfaces, and has been evolving ever since mucus was first produced.
Mucus mainly consists of huge molecular complexes called mucins that are made up of thousands of glycan sugars attached to a central protein backbone. The team showed that phages stick to these sugars, which are like a dense biological micro-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. This newly discovered defense mechanisms is likely to be pervasive, including tissues of the body such as the eye's cornea.