“My name is Legion, for we are many”: according to the Gospel of Matthew, that’s how a man possessed by demons answered when Jesus asked his name. Demonic possession is supernatural claptrap, but science has nevertheless revealed no end of proofs that we are not alone in our own skins. Bacteria swarm through our mouths and intestines; fungi and yeasts take hold where they can in our moist, warm places; parasitic animals and microbes homestead in our blood and tissues.
As explained so vividly by Carl Zimmer and others, the chemical signals that these organisms release into our bodies can also have much more subtle effects, not just on our health but on our behavior. Toxoplasma gondii parasites make their host rats less afraid of cats, which Toxo must also infest to complete their life cycle. No need to stop there, however. Certain bacteria change the sexual preferences of their fruit fly hosts, intestinal flora alter brain development in mice, and yogurt-culture bacteria can seemingly reduce their host’s stress behaviors—to choose just a few discoveries that Ed Yong has described recently.
The sources of those chemical signals don’t have to stop at our skins, however, and they needn’t have anything to do with symbiosis, as an intriguing paper from September underscored. In Cell Research, scientists at Nanjing University in China reported that some of the regulatory molecules called microRNAs found in foods can survive digestion and change gene expression in the creatures that eat them—including humans.