Simple SummaryLawsonia intracellularis (LI)is a pathogenic bacterium in pig ileitis, which will cause the thickening of ileal mucosa and slow and uneven growth in pigs. Currently, the global positive rate of colitis is gradually increasing. It is estimated that the annual loss per infected pig exceeds 3–11 dollars, seriously increasing the cost of breeding production. Due to the high cost of commercial vaccines and immune function acquired three weeks later, there are fewer vaccines used in actual production. This experiment investigated the effects of probiotic ferment added to the diet on the growth performance, digestion, absorption, and intestinal function of piglets infected withLI, to explore the efficiency and mechanism of probiotic ferment in alleviatingLIinfection in piglets. The results suggest that probiotic ferment can reduce the colonization ofLIin the ileum, improve intestinal damage, barrier function and microbiota structure, and enhance digestive enzyme activity and nutrient transport proteins expression, thereby improving piglet growth performance, which has the effect of preventing ileitis in pigs, to provide a scientific basis for the prevention of pig ileitis and its application in the aquaculture industry.Abstract(1) Background:Lawsonia intracellularis(LI) is an obligate intracellular Gram-negative bacterium that causes porcine ileitis. Pigs infected withLIhave severe ileal lesions and show symptoms of diarrhea, indigestion, and growth retardation. Previous studies found that probiotic ferment (FAM) improved the growth performance, gut barrier, and function in piglets. Therefore, we aimed to reveal the mechanism that FAM alleviates negative performance inLI-challenged piglets by characterizing the changes in intestinal integrity, function, and gut microbiota following FAM supplementation. (2) Methods: Twenty-four healthy piglets were randomly allotted to four treatments. Three groups were challenged withLI; both FAM addition and vaccination were performed to explore their positive effects onLI-infected piglets. (3) Results: Piglets infected withLIshowed lower growth performance and typical pathological symptoms. Moreover, microscopic images showed that observed intestinal morphological damage could be repaired by FAM and vaccine. To explore the digestion of nutrients in piglets, both digestive enzyme activity and ileal transporter expression were performed to reveal the promoting effect of additives. Reduction ofLIcolonization intervention by FAM could also ameliorate abnormal differentiation and function of intestinal epithelial cells and alleviate severe inflammatory responses in piglets. Regarding the gut microbiota, both the structure and function of the ileal and colonic microbiota were altered following FAM supplementation. (4) Conclusions: In conclusion, probiotic ferment can reduce the colonization ofLIin the ileum, improve intestinal damage, barrier function and microbiota structure, and enhance digestive enzyme activity and nutrient transport proteins expression, thereby improving piglet growth performance, which has the effect of preventing ileitis in pigs.