〔Abstract〕 Objective To analyze the relationship between microorganisms and endodontic disease by using 16 S rDNA sequencing to compare the composition of the microbial community in the root canals of teeth with pulpitis and apical periodontitis.Methods Clinical samples were collected from teeth requiring root canal treatment.The total DNA of the bacteria in the samples and the gene fragments of the V3-V4 highly variable region on the 16S rDNA fragments were amplified through PCR.After sequencing by NovaSeq,statistical and bioinformatic analysis,including phylogenetic analysis,diversity analysis and analysis of group differences,were performed.Results In total,6 teeth with pulpitis and 7 teeth with apical periodontitis were collected,and a total of 8 510 OTUs were obtained after next-generation sequencing,and the analysis of bacterial diversity showed that the difference between pulpitis and apical periodontitis in terms of the composition of the bacterial flora was statistically significant(P<0.05).In particular,the relative abundance of Proteobacteria,Acidobacteriota and Actinobacteriota phylum was significantly higher in the roots of teeth affected by pulpitis than apical periodontitis.The relative abundance of Bacteroidota phylum and Synergistota phylum was significantly higher in the root canals of teeth with apical periodontitis.Conclusion There is a complex diversity of infecting microorganisms in the root canals of teeth affected by endodontic diseases.The microbial communities in the infected root canals of pulpitis and apical periodontitis show some differences,and changes in the microbial composition of the root canals may be associated with the development of endodontic diseases.