Clearance of experimental cutaneous Staphylococcus aureus infections in mice.

Staphylococcal skin infections are quite common in human patients. These infections often clear spontaneously, but may also progress locally and/or disseminate to cause serious and sometimes fatal deep infections. The present studies were undertaken to examine the clearance phase of experimental cutaneous Staphylococcus aureus infections in a mouse model system. Previous work in this system has shown that staphylococci applied to the skin rapidly disseminate to the spleen and kidney. In the present experiments the bacteria were found to persist at the skin infection site at a time (8 days after inoculation) when they had disappeared from the spleen and kidney. Examination of the infected skin at earlier times revealed rapid (within 6 h) invasion into the stratum corneum, stratum Malpighii, and dermis, but subsequent redistribution of bacteria (at 1-2 days) to more superficial sites, particularly crusts located just above the skin surface. The crusts seen in these infections were of two distinct types, which were termed type 1 and type 2. Type 1 crusts appeared first, consisted of bacteria, inflammatory cells, and debris, and developed over an intact epidermis. Type 2 crusts arose from the process of dermal necrosis previously reported to take place at 2 days in this model system. In the latter situation the bacteria were not really cleared from the epidermis and dermis; rather those layers were transformed into a superficial crust that contained the bacteria. Deep hair follicle infections in the dermis were found in these infections, but they did not persist and did not seem to be a reservoir for organisms in the dermis. Resolution of these experimental infections appeared to involve redistribution of invading bacteria to more superficial locations in crusts above the skin surface, marked proliferation of the epidermis, loss of the bacteria-laden crusts from the skin, and eventual healing of the cutaneous damage.