Abstract: Objective To explore the changes of porphyromonasgingivalis and its virulence factor gingipain K during orthodontic treatment.Methods Totally 45 orthodontic healthy teenagers were randomly chosen from the orthodontics department.Givgival crevicular fluid before orthodontic treatment,in the first, second, third and sixth month after treatment were collected simultaneouslyat each time point.16S rDNA PCR technique was used to detect porphyromonasgingivalis and its virulence factor gingipain K.SPSS 17.0 software package was used to analyze the data.The detection rates of porphyromonasgingivalis and gingipain K between groups were compared by χ2. The correlation of the detection rates and gingival inflammation was analyzed by Spearman.Results Porphyromonasgingivalis before treament was significantly different form that in the first month after treatment (P<0.05), and demonstrated extremely significant difference from that in the second and third month(P<0.01). Difference was also found between the first month after treatment and the second month after treatment.As for gingipain K,there was no difference before treatment,the first month after treatment and the sixth month after treatment，while statistical difference was found between the second month after treatment and the third month after treatment（P＜0.05）.Since the third month after treatment,the rate of porphyromonasgingivalis and gingipain K had decreased，and in the sixth month after treatment,it approached the level before treatment.Conclusions During the early process of orthodontic treatment,porphyromonasgingivalis and gingipain K may increase and can lead to inflammatory reaction.Started from the second month after treatment,it began to decline and it basically approached the level before treatment in the sixth month.

Key words: orthodontictreatment, porphyromonasgingivalis, gingipain K, gingivitis