Abstract: The purpose of this experiment was to investigate the effects of Bacillus subtilis (BS) on intestinal immunity, intestinal tissue morphology, and intestinal barrier of broilers challenged with lipopolysaccharide (LPS). A total of 240 one-day-old healthy AA broilers (males and females in half) were randomly divided to 4 treatments as a 2×2 factorial design. The main factors were diets (a basic diet or a experimental diet added with 200 g·t^-1 BS) and LPS stress (oral administration of LPS solution or saline),and randomly divided into: 1) The basal diet+normal saline group; 2) The basal diet+LPS group; 3) The experimental diet+ normalsaline group; 4) The experimental diet+LPS group, respectively, a total of 4 groups, each group composed of 6 replicates with 10 broilers per replicate. The trial lasted for 28 days, and the experimental period was divided into three stages, including pre-stress (1 to 14 days), LPS stress (15 to 21 days) and recovery (22 to 28 days). The results showed as follows: l ) On the 21st day, there was a significant interaction between BS diet and LPS stress on the expression of IL-1β and TLR4 mRNA in duodenum(P<0.05), MyD88 mRNA in jejunum(P<0.05), and TNF-α mRNA in ileum(P<0.05). Without LPS stress, none of the above indicators were significantly affected by BS diet (P>0.05). Under LPS stress, BS significantly down-regulated the mRNA expression levels of those genes(P<0.05).2)On the 21st day, there was an significant interaction between BS and LPS on VH and V/C ratio in duodenum and CD in jejunum (P<0.05). BS diet significantly increased VH and V/C ratios with or without LPS stress in duodenum. Without LPS stress, BS diet significantly reduced CD of jejunum (P<0.05); Under LPS stress, BS diet had no significant effect on CD of each intestinal segment(P>0.05). On the 28th day, there existed significant interaction between BS and LPS on VH and V/C ratio in jejunum. Without LPS stress, BS diet has no significant effect on VH or V/C ratio of jejunum (P>0.05); Under LPS stress, the BS diet significantly increased jejumum VH and V/C ratio (P<0.05).3)LPS stress significantly reduced the mRNA expression of Claudin-1, Occludin, ZO-1 and MUC2 in the intestine at 21 days. BS diet significantly increased the expression of Occludin and ZO-1 mRNA in duodenum, as well as Occludin, ZO-1 and MUC2 mRNA in jejunum. The results showed that dietary BS supplementation significantly decreased the expression of intestinal proinflammatory cytokines, increased the expression of tight junction proteins Claudin-1, Occludin, ZO-1 and MUC2 mRNA, increased intestinal VH and V/C ratio, effectively alleviated the damage of immune stress on intestinal morphological structure, and promoted the growth and development of small intestine,maintains intestinal health. Dietary BS can alleviate the damage of intestinal morphology and structure caused by LPS stress, promote the growth and development of small intestine, and maintain intestinal barrier.

Key words: Bacillus subtilis, broilers, lipopolysaccharide, intestinal barrier