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.