Limited vertical transmission of microbiomes through the chorioallantoic membrane affects intestine development and metabolic pathways in broiler embryos.

The chorioallantoic membrane (CAM) in chickens is a vascularized structure that plays a crucial role in embryonic growth and development by facilitating gas exchange, nutrient transport, and waste removal. Understanding the microbial diversity and immune status of the CAM can provide valuable insights into the gut microbiome and post-hatch growth. Embryonic thermal manipulation (TM) represents an effective approach for enhancing broiler production. This study investigated the effects of TM on CAM and intestinal microbiota composition, metabolic pathways, and immunity-related gene markers. For embryonic TM, fertile Cobb 500 eggs (n = 600) were incubated in three incubators at a standard temperature of 37.5 °C with 55% relative humidity (RH) for the first 11 embryonic days (ED). After candling, on ED 12, eggs were allocated to two groups: (1) Control (n = 236) maintained at 37.5 °C and 55% RH for 24 h/d until hatch day (ED 21), and (2) TM group (n = 238) subjected to 38.5 °C and 55% RH for 12 h/d from ED 12 to ED 18, followed by standard temperature from ED 19 to ED 21 in two incubators with automatic temperature control, 55% RH. Two newly identified bacterial genera, Phenylobacterium and Limnobacter, were found in CAM. Three previously unreported bacterial genera, Phyllobacterium, Flavobacterium, and Rubelimicrobium, were found in the intestine. The bacterial genera Achromobacter, Stenotrophomonas, Lactobacillus, and Staphylococcus showed the highest overlap in the CAM and intestines. TM significantly increased (p < 0.05) CAM and intestines' microbial diversity (alpha and beta diversity) and metabolic microbial pathways. The mRNA expressions of IL1B, IL10, IL12, and IL18 were significantly higher (p < 0.05) in the D18TM compared to the other treatment groups in the CAM. The study provides initial evidence of limited vertical transmission of microbiomes through the CAM into the intestine, along with characterization of CAM microbiomes. Embryonic TM significantly influenced both the diversity of CAM and intestinal microbiota, their associated metabolic pathways, and CAM immunity.