Identification of bacterial signals that modulate enteric sensory neurons to influence behavior in C. elegans
The bacterial microbiome influences many aspects of animal health and disease. Some bacteria have beneficial functions, for example providing nutrients, whereas others act as pathogens. These bacteria are sensed by host cells to induce adaptive changes in physiology and behavior. While immune and intestinal cells detect bacterial signals through well-characterized mechanisms, recent studies indicate that neurons can also directly sense bacterial signals. However, the bacterial sensory mechanisms in neurons are less well understood. In the nematode Caenorhabditis elegans, the enteric sensory neuron NSM innervates the pharyngeal lumen and is directly activated by bacterial food ingestion; in turn, NSM releases serotonin to induce feeding-related behaviors. However, the molecular identities of the bacterial signals that activate NSM are unknown. To identify these signals, we systematically probed bacterial macromolecules from nutritive bacteria using biochemical approaches and GC-MS identification. We find that polysaccharides from gram-positive and gram-negative bacteria are sufficient to activate NSM. We further identify peptidoglycan from gram-positive bacteria as a specific component capable of activating NSM. NSM responses to polysaccharides require the acid-sensing ion channels DEL-3 and DEL-7, which localize to its sensory dendrite in the pharyngeal lumen. Ingestion of bacterial polysaccharides enhances feeding rates and reduces locomotion, matching the known effects of NSM on behavior. We also examine bacterial signals from pathogenic bacteria that can infect and kill C. elegans. This approach identifies prodigiosin, a metabolite from pathogenic Serratia marcescens, as a bacterial cue that prevents NSM activation by nutritive bacterial signals. This study identifies molecular signals that underlie neuronal recognition of nutritive bacteria in the alimentary canal and competing signals from a pathogenic bacterial strain that mask this form of recognition.