Many animals coexist in a relationship with the diverse communities that live in and on them. Known as microbiomes, these communities are made up of fungi, viruses, and bacteria. Researchers know that several bacteria thatlive in the guts of hosts produce bioactive molecules such as neurotransmitters. Scientists have hypothesized that those compounds can alter a host's mood or behavior. Now, working with the small worm Caenorhabditis elegans, Piali Sengupta's group at Brandeis University has not only shown that a bacteria in the worm's gut microbiome quantitatively alters behavior, but also the molecular basis of that alteration (Nature 2020, DOI: 10.1038/s41586-020-2395-5). The bacterium responsible is a strain of Providencia alcalifaciens called P. alcalifaciens JUb39, which produces the neurotransmitter tyramine. C. elegans converts this compound to octopamine, which then activates specific neurons in the worm and decreases its aversion to bacterially produced volatiles such as octanol. This means the worm changes its choice of food to benefit P. alcalifaciens and maybe also the worm.