摘要Background Body weight is an important indicator of the overall health and production efficiency in broiler chick-ens.In broiler houses,body weight of chicks is variable despite the same genetics,hatching and feeding practices within a production system.The objective of this study was to investigate the intestinal microbiota and bile salt hydro-lase(BSH)activity in slow and fast growing broiler chickens,which belonged to the 10th and 90th percentile body weight groups,respectively.Methods A total of 300 Ross 308 broiler chickens(100 per cohort from three independent cohorts)were selected and mucosal samples from the jejunum,ileum,and cecum were collected at day of arrival,11 and 25(n=450).Then,bacterial counts,16S rRNA amplicon sequencing,species specific real-time qPCR,as well as BSH activity were analyzed.Results Results of bacterial counts showed no significant difference between slow and fast growing cohorts(P＞0.05),but they tended to be higher in the slow growing chickens in all measured bacterial groups in cecum.The 16S rRNA amplicon sequencing revealed higher relative abundance of E.coli-Shigella(71.3％-79.8％)at day of arrival,while the most abundant microorganisms at d 25 was Candidatus Arthromitus(slow:44.5％;fast:27.4％)in small intestine.qPCR results indicated significant differences in bacterial populations between the slow and fast growing chickens,especially higher total bacteria,Enterococcus,and Clostridium cluster I in the slow growing chick-ens at d 25.BSH activity was higher in the slow growing chickens than the fast growing chickens[slow:0.476 △OD/protein(μg/mL);fast:0.258 △OD/protein(μg/mL);P＜0.0001],and correlation analysis highlighted associations between BSH activity,body weight,feed intake,body weight gain,and bacterial counts.Conclusions We postulate that high total bacteria and Enterococcus abundance are associated with high BSH activ-ity,impacting low feed intake and body weight gain,ultimately resulting in separation into slow and fast growing birds.The findings of this study contribute to understanding the relationship between gut microbiota,BSH activity,and host physiology in broiler chickens,with potential implications for poultry production.