NITRA aims to evaluate the potential of dietary tryptophan supplementation to enhance the efficacy of dip vaccination in rainbow trout (Oncorhynchus mykiss) against Yersinia ruckeri, a major bacterial pathogen responsible for significant economic losses in aquaculture. Tryptophan is an essential amino acid increasingly recognized for its immunomodulatory functions, including the regulation of immune responses, stress resilience, and mucosal health. The study will investigate how dietary tryptophan influences vaccine-induced protection by assessing immune responses, survival, and resistance to infection following bacterial challenge. In parallel, it will characterize changes in the skin and gut microbiomes, exploring their relationship with immune function and vaccine performance. By integrating nutritional intervention, vaccination, microbiome profiling, and pathogen challenge, the project seeks to generate mechanistic insights into the interactions between diet, host immunity, and microbial communities. The results will advance current knowledge on functional amino acids in aquaculture and support the development of innovative functional feeds capable of improving vaccine efficacy, enhancing fish health and welfare, reducing disease-related losses, and contributing to more sustainable and resilient aquaculture production systems.