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This research focused on the effects of different doses of Bacillus subtilis KN-42 on the growth performance,diarrhea incidence,faecal bacterial flora,and the relative number of Lactobacillus and Escherichia coli in faeces of weaned piglets to determine whether the strain can serve as a candidate antimicrobial growth promoter.A total of 360 piglets (initial BW 7.14 ± 0.63 kg) weaned at 26±2 days of age were randomly allotted to 5 treatment groups (4 pens per treatment with 18 pigs per pen) for a 28-day trial.Dietary treatments were basal diet without any antimicrobial (negative control;NC),basal diet supplemented with 120 mg/kg feed of neomycin sulfate (positive control;PC) and basal diet supplemented with 2 × 109 (L),4 × 109 (M) and 20 × 109 (H) CFU/kg feed of B.subtilis KN-42.During the overall period,ADG and feed efficiency of piglets were higher in groups PC,M,and H than those in group NC (P<0.05),and all probiotics and antibiotics groups had a lower diarrhea index than group NC (P<0.05).The 16S rDNA gene-based methods were used to analyze faecal bacterial flora on day 28 of experiment.The result of denaturing gradient gel electrophoresis (DGGE) analysis showed that supplementation of B.subtilis KN-42 to the diet changed the bacterial communities,with a higher bacterial diversity and band number in group M than in the other four groups.Real-time PCR analysis showed that the relative number of Lactobacillus were higher in groups PC and H than in group NC (P<0.05),and the supplemented B.subtilis KN-42 to the diet also reduced the relative number of E.coli (P<0.05).These results suggest that dietary addition of B.subtilis KN-42 can improve the growth performance and gastrointestinal health of piglets.