Analysis of epidermal entry in experimental cutaneous Bacillus anthracis infections in mice

Cutaneous infection is the most common form of human anthrax, but little is known of Bacillus anthracis-epidermal interactions. To study the latter, we used experimental inoculations of B. anthracis Sterne spores onto mouse flank skin. In DBA/2 mice (a sensitive strain) 107 spores injected intradermally or applied under occlusive dressings to abraded skin produced ipsilateral inguinal edema and rapid death. Epicutaneous application to shaved-only skin produced edema and death in most animals, but at longer times. Mortality after inoculation onto abraded skin was less in C57BL/6 mice (a relatively resistant strain). Inoculations onto shaved-only skin immunized C57BL/6 mice, and they survived later intradermal spore injections. Histology revealed massive organism proliferation in remaining epidermis and hair follicles of inoculated abraded skin, but less growth in the dermis itself. Conversely, no foci could be located by microscopic examination after inoculation onto shaved-only skin. High-dose nonocclusive dressing inoculations onto unshaved skin in DBA/2 mice revealed small numbers of infective foci, all in hair follicles. These results suggest that epidermal damage may increase infection susceptibility to B. anthracis of hair follicle contents and remaining epidermal remnants; the findings also indicate that access may occur through hair follicles and the denuded dermis.