Background  Streptococcus infection can induce and aggravate psoriasis. Imiquimod-induced psoriasis-like dermatitis animal model is the most widely used model at present. However, its application value is hampered by its short duration and regression.

  Objective  To observe whether Streptococcus can modify the imiquimod-induced psoriasis-like dermatitis animal model.

  Methods  C57BL/6 mice were randomly divided into Streptococcus group (GAS+IMQ), IMQ group (PBS+IMQ) and control group (PBS+VAS), with 15 mice in each group. On the first day, Streptococcus bacteria solution was added into the nasal cavity of the Streptococcus group, and the same amount of PBS solution was added into the control group and IMQ group. Imiquimod cream was applied on the third day for modeling in the Streptococcus group, IMQ group, and vaseline cream for the control group. The continued observation was performed for another 21 days. From the third day, skin lesions were photographed daily, and the PASI score was performed. Three mice in each group were randomly sacrificed on day 0, 3, 10, 17 and 24, then skin tissues of each group were collected for histopathological examination and the expression level of Ki-67 in skin tissue was detected by immunohistochemistry. The serum levels of IL-17, IL-6, and TNF-α on day 24 were determined by enzyme-linked immunosorbent assay (ELISA).

  Results  Psoriasis-like dermatitis lesions were found in both the Streptococcus group and IMQ group after modeling, and PASI scores reached the peak on day 10. The severity of psoriasis-like dermatitis lesions in IMQ group gradually decreased and vanished on day 24, and it was severer in the mice of Streptococcus group than those in the IMQ group (P＜0.05). The psoriasis-like dermatitis lesions of the Streptococcus group were maintained on day 24 in mice of Streptococcus group. Skin histopathology showed that munro microabscess, spinous layer hypertrophic, and dermal inflammatory cell infiltration were observed in both Streptococcus group and IMQ group. Compared with the IMQ group and the control group, the epidermal thickness of the Streptococcus group was higher (P＜0.05), so did the positive expression of Ki-67 in skin lesions (P＜0.05). Compared with the control group and the IMQ group, the serum levels of IL-17, IL-6, and TNF-α in the Streptococcus group increased (all P＜0.05).

  Conclusion  Streptococcus can prolong the maintenance time of the imiquimod-induced psoriasis-like dermatitis animal model, which is more typical and may be a feasible and effective modified animal model.