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Background:The relationship between diet and intestinal microbiota and mucin composition appears to be fundamental for poultry gut health. The effects of insect meal (whose role as altative feed ingredient is now well recognized) on gut microbiota and mucin composition have recently been reported in Tenebrio molitor-fed free-range and broiler chickens, but no data are currently available for Hermetia illucens (HI)-fed broilers. The present study evaluated the effects of dietary HI meal inclusion on cecal microbiota and intestinal mucin composition of broiler chickens. Results:A total of 256 male broiler chickens were allotted to 4 dietary treatments (control diet [C] and 5％, 10％and 15％HI meal inclusion, with 8 replicate pens/treatment and 8 birds/pen) and slaughtered at 35 d of age (2 animals/pen, 16 birds/diet). The cecal microbiota assessment by 16S rRNA amplicon based sequencing showed lower alpha diversity in HI15 chickens (Shannon, P<0.05) and higher beta diversity (Adonis and ANOSIM, P<0.001) in birds fed HI diets than C. Furthermore, HI15 birds displayed significant increase of the relative abundance of Proteobacteria phylum (False Discovery Rate [FDR]< 0.05) when compared to HI10. L-Ruminococcus (Ruminococcus from Lachnospiraceae family), Faecalibacterium, Blautia and Clostridium genera were found to be characteristic of HI5 cecal microbiota (FDR<0.05), while broiler chickens fed HI10 and HI15 diets were characterized (FDR<0.05) by Lactobacillus and Ruminococcus (HI10) and Bacteroides, Roseburia and Helicobacter genera (HI15). Periodic-acid Schiff, Alcian Blue pH 2.5 and high iron diamine staining on small and large intestine also demonstrated lower mucin staining intensity in the intestinal villi of HI10 and HI15 birds than C (P<0.05). Conclusions:Dietary HI meal utilization at low inclusion levels (i.e., 5％) positively influenced either the cecal microbiota or the gut mucin dynamics in terms of selection of potentially beneficial bacteria and increase in villi mucins. However, high inclusion levels (in particular the 15％) may have a negative influence in terms of partial reduction of microbial complexity, reduction of potentially beneficial bacteria, selection of bacteria with mucolytic activity and decrease in villi mucins.