Antibacterial effect of β-thujaplicin on staphylococci isolated from atopic dermatitis: relationship between changes in the number of viable bacterial cells and clinical improvement in an eczematous lesion of atopic dermatitis

β-Thujaplicin (hinokitiol) is a tropolone-related compound purified from the wood of Chamae- cyparis obtusa, Sieb. et Zucc. and Thuja plicata D. Don. All Staphylococcus aureus isolates were inhibited by β-thujaplicin with MICs of 1.56-3.13 mg/L. However, a paradoxical zone phenom- enon occurred, with each isolate producing regrowth at higher β-thujaplicin concentrations. Other antimicrobial agents showed a wide range of MICs. The combination of β-thujaplicin and zinc oxide inhibited the paradoxical zone phenomenon, and enhanced killing activity against clinically isolated staphylococci. Large numbers of viable bacterial cells, especially S. aureus cells, were detected in the skin surface of atopic dermatitis, in comparison with those in healthy volunteers. The number of cells increased as the severity of the skin condition worsened. Topical application of β-thujaplicin resulted in a reduction in the number of bacterial cells on the skin surface, and an improvement in skin condition after treatment. The results of this study suggest that the degree of reduction in the number of viable bacterial cells in an eczematous lesion of atopic dermatitis is related to the degree of improvement in skin condition.