Abstract: ObjectiveTo examine the relative content changes of Porphyromonas gingivalis(P.gingivalis)inoculated to subgingival of first maxillary molars of the rats in different times and the changes of virulence modulating gene A (vim A)by establishing the rat periodontitis model.MethodsWe established a rat experimental periodontitis model by ligaturing orthodontic steel wire and feeding bacteria.The subgingival plaque was collected,then the DNA was extracted and amplified with PCR using specific primer.We analyzed the genetic changes of vim A in different stages of the rat periodontitis using SPSS11.0 with two independent t test.ResultsThe relative contents of P.gingivalis of experimental group in 4 weeks and 8 weeks were significantly different and obviously higher than those of the control group,while the relative contents of vim A for both of the experimental group and healthy controls increased.ConclusionVim A gene may relate to the pathogenicity of periodontitis,but have no direct effect on the severity of periodontal disease.