Acid-treatedStaphylococcus aureusinduces acute silkworm hemolymph melanization

Abstract

AbstractThe skin microbiome maintains healthy human skin, and disruption of the microbiome balance leads to inflammatory skin diseases such as folliculitis and atopic dermatitis.Staphylococcus aureusandCutibacterium acnesare pathogenic bacteria that simultaneously inhabit the skin and cause inflammatory diseases of the skin through the activation of innate immune responses. Silkworms are useful invertebrate animal models for evaluating innate immune responses. In silkworms, phenoloxidase generates melanin as an indicator of innate immune activation upon the recognition of bacterial or fungal components. We hypothesized thatS. aureusandC. acnesinteract to increase the innate immunity-activating properties ofS. aureus. In the present study, we showed that acidification is involved in the activation of silkworm hemolymph melanization byS. aureus. Autoclaved-killedS. aureus(S. aureus[AC]) alone does not greatly activate silkworm hemolymph melanization. On the other hand, applyingS. aureus[AC] treated withC. acnesculture supernatant increased the silkworm hemolymph melanization. AddingC. acnesculture supernatant to the medium decreased the pH.S. aureus[AC] treated with propionic acid, acetic acid, or lactic acid induced higher silkworm hemolymph melanization activity than untreatedS. aureus[AC].S. aureus[AC] treated with hydrochloric acid also induced silkworm hemolymph melanization. The silkworm hemolymph melanization activity ofS. aureus[AC] treated with hydrochloric acid was inhibited by protease treatment ofS. aureus[AC]. These results suggest that acid treatment ofS. aureusinduces innate immune activation in silkworms and thatS. aureusproteins are involved in the induction of innate immunity in silkworms.