Comparison of sodium azide- and cooling-based immobilization methods for Caenorhabditis elegans chemotaxis assays

Chemotaxis assays are widely used to quantify sensory-driven behavioral responses in Caenorhabditis elegans . Sodium azide is commonly used to immobilize animals at assay endpoints, although concerns have been raised that diffusion beyond the endpoint region could affect behavior before a choice is completed. We compared sodium azide-mediated immobilization with post-assay cooling using the repellent 1-octanol and attractant diacetyl. Wild-type animals showed robust avoidance of 30% 1-octanol and attraction to 0.5% diacetyl with either method, across multiple assay durations, and with or without worm food Escherichia coli OP50. These results support post-assay cooling as an alternative endpoint-scoring method.