Abstract: Objective　 This study intends to investigate the distribution of Porphyromonas gingivalis and Porphyromonas gingivalis’ two kgp gene type in primary periapical infection root canal, and to analyze the correlation between the genotypes and clinical symptoms. Methods　 We collect 190 cases of primary periapical infection patients in clinic. We collect sample from their infected root cannel and we extract bacteria DNA from the sample we collected. We use Polymerase Chain Reaction to determine the presence of Porphyromonas gingivalis. We get the kgp catalytic domain with the help of Polymerase Chain Reaction. Tru I enzyme digestion to the kgp catalytic domain help us to determine the Porphyromonas gingivalis gene polymorphism. After all, we use statistical methods to the analysis of the experimental data. Results　55 patients in 190 patients with primary periapical infection can be detected with Porphyromonas gingivalis. The detection rate of kgp Itype is 27.3%, while the detection rate of kgpⅡtype is 38.2%. Porphyromonas gingivalis were correlated with clinical symptoms. kgp I type were correlated with pain. Conclusions　 The kgp gene of Porphyromonas gingivalis which is in root cannel of primary periapical infection has gene polymorphism. Because some samples do not contain enough quantity of Porphyromonas gingivalis, it is sometimes difficult for method of kgp specific primers PCR to detect kgp gene. In further research, we will find more sensitive means to detect kgp gene in infected root canals.

Key words: Porphyromonas gingivalis, kgp gene, gene polymorphism, periapical infection