Production of experimental staphylococcal impetigo in mice

We produced a staphylococcal impetigo model by epicutaneous inoculation in mature mice. A strain isolated from a human impetigo was used. Five-week-old female mice (ddy-strain) were used with and without pre-treatment by cyclophosphamide (Cy) (2 mg/mouse) for 5 days. The back skin of mice was shaved by a razor blade and slightly abraded by sand paper. Bacterial suspension (1.4 x 10(7) CFU/0.05 ml) was applied on the abraded areas which were then occluded under sterile plastic plaster. Although intraepidermal blisters developed in non-Cy-treated mice, massive neutrophil infiltration obscured the changes there. Development of subcorneal bullae in Cy-treated mice inoculated with Staphylococcus aureus was first observed at 3h and enlargement of bullae was apparent at 12 h after inoculation. The bullae produced in Cy-treated mice contained numerous S. aureus bacilli. Electronmicroscopically, S. aureus cells invaded the horny layer at 1/4 h. A clear halo was seen between S. aureus cells and horny cells. S. aureus cells attached to surrounding horny cells by fibril-like structures. The halo-like spaces became larger, coalesced and then developed into an intraepidermal blister. Our new method to produce human impetigo-like blister in Cy-treated adult mice may contribute to disclosing the mechanisms of blister formation in epidermis by S. aureus. Due to the thin structure of mouse epidermis, only specimens taken earlier than 24 h after inoculation were considered appropriate.