Abstract: 　Objective To study the pathogenesis of Entamoeba gingivalis ( E.g .) and its relation to periodontal diseases.\ Methods Rats were treated with immuno-inhibitor for one week and the neck of incisor teeth of the rats was bound with steel wire. They were randomly divided into three groups: the first group was infected by E.g . in the periodontal tissue, the second group was infected by symbiotic bacteria (s.b.),and the third group was given physiological saline as control.Observation on the periodontal inflammation was made for each group of rats, and the purulent secretion from periodontal abscess was examined for living pathogens.\ Results The incidence of periodontal diseases in rats infected by E.g . was higher than that of symbiotic bacteria group and that of control ( P <0.05), the incidence of periodontal diseases in rats infected by s.b. was higher than that of control group ( P <0.05). Living pathogens were found in the abscess liquid.\ Conclusion E.g . is an opportunistic pathogen, which, together with synergistic symbiotic bacteria, can cause periodontal diseases in hosts with low immunity.

Key words: Entamoeba gingivalis, symbiotic bacteria, pathogenesis, periodontal disease