Economist: Virus Shield

The Economist


Microbiomics: A Virus Shield

Viruses help keep people disease-free

By The Economist, May 25 2013.

THE story of the microbiome—the community of tiny organisms that coexist symbiotically with people and other animals—gets weirder every day. Until now that story, which has emerged over the past few years, has seemed one of friendly collaboration. In exchange for a place to live, bacteria aid digestion and help regulate their host’s metabolism. The story’s latest chapter, though, describes a different deal: between the animal host and viruses. And in this deal, bacteria are the targets.

Jeremy Barr of San Diego State University and his colleagues, who have just published their results in the Proceedings of the National Academy of Sciences, were intrigued by the high concentrations of viruses called bacteriophages in the mucus secreted by soft-tissue membranes ranging from the outer layers of sea anemones via the skins of fish to the mouths of people. Such membranes are prime targets for pathogens, which is why so many people suffer from throat and lung infections, and upset guts. Bacteriophages (or phages, for short) parasitise bacteria…. Dr Barr wondered if they were actively working with their hosts to keep down the level of pathogenic bugs that might otherwise set up shop in the hosts’ mucus membranes.

Regardless of which type of membrane they looked at, Dr Barr and his colleagues found about four times more phages per bacterium in the mucus than were in its immediate surroundings. This suggested that phages are sticking specifically to mucus, and in an elegant series of experiments they found that this was true and that the thing on a phage which is doing the sticking is a “hypervariable” protein called Hoc.

Hypervariable proteins come in thousands of slightly different versions. The best studied is the mammalian antibody protein, immunoglobulin. Each of the versions of this protein that exist in every individual is capable of locking onto and neutralising a different invading pathogen, by interacting with different molecules on the pathogen’s surface. In phages, Dr Barr discovered, the various versions of Hoc stick to the varied branched sugar molecules characteristic of mucus.

That helps the phages, because their bacterial prey also accumulate in mucus, which is thus a rich hunting ground. And it helps the animal host, be that host sea anemone, fish or human being, by stopping any particular bacterial species running out of control—for if a species does start to multiply, which is a normal precursor to infection, there will be more phages around to attack it and multiply in their turn, bringing the uppity bug rapidly under control. The phages are thus acting as a backup immune system for their hosts, and one whose mode of operation has a surprising parallel with that of the more familiar antibody-based system.

See The Economist, Microbiomics, May 25 2013.

(Emphasis added)