Dietary 1.3-1.6 yeast β-glucans enhance immune response and disease resilience in European seabass challenged with Tenacibaculum maritimum

Introduction: Nutritional strategies have emerged as promising tools to modulate immune responses in marine fish, particularly at mucosal surfaces involved in host-pathogen interactions. Dietary immunomodulators such as yeast-derived β-glucans (BG) have gained attention for their capacity to enhance innate immune defenses, however their role in coordinating these mechanisms during bacterial infections remains incompletely understood. Methods: specifically. After four weeks of feeding with experimental diets, fish were exposed to a sub-lethal infection, and immune parameters were accessed in the skin, intestinal mucosa and at systemic level. Results: Growth performance and feed efficiency were not affected by BG supplementation. At 24 h post-infection, fish fed with the 0.12% BG diet maintained circulating leukocyte counts and displayed increased plasma lysozyme activity, indicating enhanced early immune readiness. Moreover skin immune responses showed an early upregulation of pro-inflammatory and antimicrobial related genes, followed by increased expression of regulatory cytokines at 7 days post-infection, suggesting an efficient transition from immune activation to resolution. In contrast, intestinal immune responses remained largely unchanged, indicating a compartmentalized modulation focused on the primary site of infection. Conclusion: Overall, dietary BG supplementation exerted a dose dependent immunostimulatory effect in European seabass, with 0.12% BG eliciting the most significant chances in disease resilience.