C. elegans interprets dietary quality through context-dependent serotonergic modulation

Animals sense their metabolic needs to guide foraging decisions using neuronal pathways that are only partly understood. Here, we systematically investigate how foraging in the nematode Caenorhabditis elegans is influenced by its bacterial diet, E. coli. By screening C. elegans behavior on 3983 E. coli knockout strains, we identified 22 E. coli metabolic mutants that are aversive to C. elegans in a long-term foraging assay. These include the global metabolic regulator CRP and genes affecting cysteine synthesis, vitamin B6 synthesis, and iron uptake. Serotonin, a neurotransmitter associated with feeding in many animals, allows C. elegans to distinguish wild-type E. coli from these "mediocre" diets through bidirectional signaling. Serotonin produced by the ADF serotonergic neurons supports attraction to wild-type E. coli with the serotonin receptor genes ser-4 and ser-5, whereas serotonin produced by the NSM serotonergic neurons differentially drives aversion to two mediocre diets through four serotonin receptor genes, ser-1, ser-7, mod-1, and lgc-50. Serotonin receptors act in multiple target neurons, including octopamine-producing neurons that suppress aversion across all diets. In addition, dopamine promotes aversion, in part by inhibiting octopaminergic neurons. These results reveal interactions between neuromodulatory circuits in the context-dependent evaluation of dietary quality.