摘要Two studies were conducted to investigate the effect of Bacillus amyloliquefaciens CECr 5940 (BA) as a probiotic on growth performance,amino acid digestibility and bacteria population in broiler chickens under a subclinical necrotic enteritis (NE) challenge and/or fed diets with different levels of crude protein(CP).Both studies consisted of a 2 × 2 factorial arrangement of treatments with 480 Ross 308 mix-sexed broiler chickens.In study 1,treatments included 1) NE challenge (+/),and 2) BA (1.0 × 106 CFU/g of feed) supplementation (+/-).In study 2,all birds were under NE challenge,and treatments were 1) CP level (Standard/Reduced[2％ less than standard]) and 2) BA (1.0 × 106 CFU/g of feed) supplementation(+/-).After inducing NE infection,blood samples were taken on d 16 for uric acid evaluation,and cecal samples were collected for bacterial enumeration.In both studies,ileal digesta was collected on d 35 for nutrient digestibility evaluation.In study 1,the NE challenge reduced body weight gain (BWG),supressed feed conversion ratio (FCR) and serum uric acid levels (P ＜ 0.001).Supplementation of BA increased BWG(P ＜ 0.001) and reduced FCR (P =0.043) across dietary treatments,regardless of challenge.Bacillus(P =0.030) and Ruminococcus (P =0.029) genomic DNA copy numbers and concentration of butyrate(P =0.017) were higher in birds fed the diets supplemented with BA.In study 2,reduced protein (RCP)diets decreased BWG (P =0.010) and uric acid levels in serum (P ＜ 0.001).Supplementation of BA improved BWG (P =0.001) and FCR (P =0.005) and increased Ruminococcus numbers (P =0.018) and butyrate concentration (P =0.033) in the ceca,regardless of dietary CP level.Further,addition of BA reduced Clostridium perfringens numbers only in birds fed with RCP diets (P =0.039).At d 35,BA sup-plemented diets showed higher apparent ileal digestibility of cystine (P =0.013),valine (P =0.020),and lysine (P =0.014).In conclusion,this study suggests positive effects of BA supplementation in broiler diets via modulating gut microflora and improving nutrient uptake.