As a novel protein source, the black soldier fly (Hermetia illucens) possesses the potential to enhance fish health due to its high protein content, essential amino acids, and bioactive compounds, but the mechanisms by which defatted black soldier fly meal affects fish health remain unclear. This study aimed to evaluate the effects of partially replacing soybean meal (SM) with defatted black soldier fly larvae meal on the pathogen resistance of juvenile Nile tilapia (Oreochromis niloticus) and to explore the underlying mechanisms. Defatted black soldier fly (Hermetia illucens) meal was used to replace 0 % (CON), 15 % (H15), 30 % (H30), 45 % (H45), and 60 % (H60) of soybean meal in the diets, which were fed to Nile tilapia for 8 weeks. At the second week of the experiment, an unexpected infection was observed in the Nile tilapia, with subsequent isolation and identification confirming the pathogen as Aeromonas veronii. The results showed that defatted black soldier fly meal improved the survival rate and reduced the intestinal inflammation of Nile tilapia in H60 group compared to CON group (P <
0.05). Periodic acid-Schiff (PAS) staining of the gut indicated that a 60 % replacement of soybean meal with defatted black soldier fly meal significantly increased the number of goblet cells in the intestine (P <
0.05) and upregulated the expression level of mucin 2 (muc2) (P <
0.05). Throughout the experiment period, the survival rate in the defatted black soldier fly meal groups was higher than that in the soybean meal group, with the H60 group exhibiting the most significant improvement. In addition, defatted black soldier fly meal exhibited a promotion effect on goblet cell numbers, mucin production and mucin secretion with a dose dependent manner (P <
0.05). To identify the possible mechanism by which defatted black soldier fly meal enhanced goblet cell numbers, gut microbiome and metabolomics were conducted. The results showed that the defatted black soldier fly meal altered the composition of intestinal microbiota and increased the content of L-tyrosine in the H60 group. The effects of L-tyrosine were further identified in LS174T cells, and the results showed that L-tyrosine upregulated the expression levels of muc2 and SAM pointed domain-containing Ets transcription factor (spdef) (P <
0.05). In conclusion, the defatted black soldier fly meal improves gut health and enhances pathogen resistance, ultimately leading to the increased survival rates of Nile tilapia when exposed to A. veronii.