Abstract: Objective To explore the characteristics of changes in gut microbiota diversity in children with sepsis and its correlation with the severity of the disease. 
Methods Fifteen children who met the diagnosis of sepsis and were admitted to the Department of Intensive Care Medicine in our hospital were selected as the research subjects, and 28 infected but non-sepsis children admitted to the pediatric intensive care unit (PICU) during the same period were selected as the control group. The feces of the patients on the first day of admission was collected, to extract the microbial DNA from the feces, and the 16S rDNA gene was amplified. Through Illumina sequencing platform, species annotation, diversity, and species difference analysis were performed on the sequencing results. The pediatric risk of mortality score Ⅲ (PRISM Ⅲ) and pediatric critical illness score (PCIS) for pediatric patients were recorded. The composition of gut microbiota in the two groups was compared and their correlation with the disease score were analyzed. 
Results The PCIS score of the sepsis group ［78(8)］ was lower than that of the non-sepsis group ［90(12)］, and the PRISM Ⅲ score ［16(16)］ was higher than that of the non-sepsis group ［12(9)］ (P＜0.05). Stool samples of the two groups were sequenced and a total of 1 489 022 valid sequences were detected. The gut microbiota abundance index ［Ace index (99.458±38.948), Chao index (99.400±38.943)］, and gut microbiota diversity index ［Shannon index (3.647±.411)］ in the sepsis group were lower than those in the non-sepsis group ［Ace index (107.879±25.242), Chao index (107.893±25.238), and Shannon index (4.125±1.160)］. Principal Coordinated Analysis (PCoA) analysis showed that the distribution of microbiota in the sepsis group and non-sepsis group showed relative concentration within the group and relative separation between the groups, and the difference in microbiota structure between the two groups of stool samples was significant (P＜0.05). There were significant differences in the structure of the dominant gut microbiota between the two groups at the phylum, class, and genus levels (P＜0.05). At the phylum and class levels, Acidobacterium and Halophilic Bacteroides were positively correlated with PRISM Ⅲ scores and negatively correlated with PCIS scores (P＜0.05). 
Conclusion The diversity and abundance of gut microbiota in children with sepsis are lower than those in non-sepsis children. The proportion of enterococci in the sepsis group is significantly higher than that in the non-sepsis group, and the proportion of pathogenic bacteria increases significantly. The imbalance of gut microbiota is correlated with the severity of the disease. 

Key words: sepsis, child, gastrointestinal microbiome