Microbial influence on intestinal development and mode of action of mannan oligosaccharides in broiler chicken

The effect of intestinal microbiota and dietary supplementation of mannan oligosaccharides (MOS) on mucosal architecture and digestive physiology in broiler chicks was examined. In experiment 1, pre-sterilized eggs (Ross x Ross 308) were placed in three HEPA (high efficiency particulate air)-filtered isolator units at day 19 of incubation. Germ-free chicks in one isolator were conventionalized by exposure to cecal contents from a laying hen. Bacterial contamination occurred in one germ-free isolator such that these birds were monoassociated by a bacterium within the Acinetobacter spp. resulting in 3 categories of microbial status including germ-free (GF, n=10), conventionalized (CV, n=19) and monoassociated (Mono, n=13) birds. Dietary treatments assigned to each isolator consisted of a negative control (NC, 0 g/kg of MOS in the basal diet) and MOS (2 g/kg of MOS in the diet) resulting in a 2X3 factorial treatment arrangement. At 7 d of age, body weight was recorded and birds were killed to permit collection of visceral organs, intestinal tissues and cecal contents. Body weight, relative length of small intestinal segments and relative bursa weight were significantly increased in CV birds. These birds also had increased crypt depth and lamina propria area. Dietary MOS increased villus height and villus surface area in CV birds compared with GF and Mono birds. Transcripts for all housekeeping genes tested in ileal tissue were increased by MOS such that transcripts were normalized to unit mass of total RNA. In comparison to birds fed the NC diet, MOS significantly increased the abundance of proliferative cell nuclear antigen (PCNA), toll-like receptor (TLR)-4, avian β-defensin (GAL)-6, interleukin (IL)-8, peptide transporter 1 (PepT1) and sodium-dependent glucose transporter (SGLT)-1 transcripts in ileum per unit total RNA. However, the effect of microbial status on selected gene expression profiles was surprisingly limited. A second experiment was conducted to confirm the conventionalization protocol produced a complex microbiota similar to conventionally reared birds. Twenty day-old broiler chicks (Ross x Ross 308) were assigned to one of two wire-floored battery cages provided the NC and MOS diets ad libitum and killed at 7 d of age. Terminal restriction fragment length polymorphism (TRFLP) analysis demonstrated that microbial diversity indices (Richness, Evenness, Shannon, and Simpson) were greater in conventionalized gnotobiotic birds compared to the conventionally reared birds confirming a successful conventionalization strategy in the gnotobiotic trial. These studies demonstrate that under good hygienic conditions, CV chicks thrive similar to GF animals. Based on responses to MOS observed in GF birds, evidence indicates that MOS, independent of changes in microbial composition, directly modifies host response parameters including innate immune activation, digestive and absorptive function