Abstract: 【Objective】 The objective of this study was to reveal the characteristics of fecal microbiota and metabolites of calves with diarrhea. 【Method】 According to the fecal score and clinical symptoms, the suckling calves within 30 days of age were divided into healthy group (CK) and diarrhea group (D).The fecal microbiota of calves in CK and D groups were analyzed using 16S rRNA amplification sequencing technology.The non-targeted metabolome analysis was performed to screen differential metabolites in the feces of CK and D groups through LC-MS, and the differential metabolites were conducted for KEGG pathway enrichment analysis.The Spearman correlation analysis was performed on fecal microbiota and differential metabolites in feces. 【Result】 Compared with CK group, the OTUs and Chao1 indexes of fecal microbiota in D group were extremely significantly increased (P＜0.01), and the Shannon and Simpson indexes were extremely significantly decreased (P＜0.01).The relative abundances of Fusobacteriota，Proteobacteria, Acidobacteriota, Chloroflexi and 9 bacteria genera that including Escherichia-Shigella, Fusobacterium and Clostridium_sensu_stricto_1, etc.were extremely significantly or significantly increased in D group (P＜0.01 or P＜0.05), while the relative abundances of Actinobacteriota, Firmicutes, Euryarchaeota and 11 bacteria genera that including Faecalibacterium, Subdoligranulum, Olsenella and Bifidobacterium, etc.were extremely significantly or significantly decreased in D group (P＜0.01 or P＜0.05).Fusobacteriota, Proteobacteria, Fusobacterium, Escherich-Shigella, Fusobacterium mortiferium and Escherichia coli were biomarkers in the feces of calves with diarrhea.A total of 54 different metabolites were screened from the feces of CK and D groups.PC(16∶1(9Z)/16∶1(9Z)) was enriched into the glycerophospholipid metabolism pathway, which was significantly positively correlated with Fusobacterium (P＜0.01) and significantly or extremely significantly positively correlated with Collinsella, Megasphaera and Olsenella(P＜0.05 or P＜0.01).Dethiobiotin was enriched into the biotin metabolic pathway.It was significantly negatively correlated with Fusobacterium (P＜0.01), while significantly positively correlated with Subdoligranulum, Megasphaera and Rolstonia (P＜0.05).Dihydrobiopterin and dihydroneopterin triphosphate enriched into the folate biosynthesis pathway were extremely significantly or significantly negatively correlated with Fusobacterium and Peptestreptococcus (P＜0.01 or P＜0.05), which extremely significantly or significantly positively correlated with Subdoligranulum, Megasphaera, Olsenella, Bifidobacterium, Prevotella, Collinsella, Faecalibacterium, and Parabacteroides (P＜0.01 or P＜0.05). 【Conclusion】 The richness, diversity, species composition of fecal microbiota and the composition of fecal metabolites were significantly changed in calves with diarrhea.Theglycerophospholipid metabolism, biotin metabolism and folate biosynthesis were affected in the gut of calves with diarrhea.The dysbiosis of intestinal microbiota of calves was closely related to the significant changes of fecal metabolites.

Key words: suckling calves; calf diarrhea; fecal microbiota; fecal metabolites