Behavioral avoidance of pathogenic bacteria by Caenorhabditis elegans

• Caenorhabditis elegans has innate and learned behavioral responses to pathogenic bacteria.
• The nervous system mediates host–pathogen interactions through chemosensation.
• Conserved pathways control both innate immunity and behavioral responses to pathogens.
• Natural genetic variation influences pathways mediating behavioral avoidance.

The simple animal host Caenorhabditis elegans utilizes its nervous system to respond to diverse microbial cues, and can engage in a protective behavioral avoidance response to environmental pathogens. This behavior bears hallmarks of an immune response, with sensors and recognition systems that trigger a protective response following a learning experience. Neuronal circuits required for aversive learning have been defined, revealing conserved signaling modules with dual roles in immunity and neuronal responses to pathogenic bacteria. Identification of natural polymorphisms that modulate avoidance behavior has enabled an improved understanding of host–microbe interactions at the molecular level. We review here these findings and discuss how the microbial cues and host responses defined in C. elegans may provide insight into evolutionarily diverse host–microbe interactions.