Ceramidase activity in bacterial skin flora as a possible cause of ceramide deficiency in atopic dermatitis

A marked decrease in the content of ceramide has been reported in the horny layer of the epidermis in atopic dermatitis (AD), This decrease impairs th e permealbility barrier of the epidermis, resulting in the characteristic d ry and easily antigen-permeable skin of AD, since ceramide serves as the ma jor water-holding molecule in the extracellular space of the horny layer. O n the other hand, the skin of such patients is frequently colonized by bact eria, most typically by Staphylococcus aureus, possessing genes such as tho se for sphingomyelinase, which are related to sphingolipid metabolism. We t herefore tried to identify a possible correlation between the ceramide cont ent and the bacterial flora obtained from the skin of 25 patients with AD v ersus that of 24 healthy subjects, using a thin-layer chromatographic assay of the sphingomyelin-associated enzyme activities secreted from the bacter ia. The findings of the assay demonstrated that ceramidase, which breaks ce ramide down into sphingosine and fatty acid, was secreted significantly mor e from the bacterial flora obtained from both the lesional and the nonlesio nal skin of patients with AD than from the skin of healthy subjects; sphing omyelinase, which breaks sphingomyelin down into ceramide and phosphorylcho line, was secreted from the bacterial flora obtained from all types of skin at similar levels for the patients with AD and the healthy controls. The f inding that the skin of patients with AD is colonized by ceramidase-secreti ng bacteria thus suggests that microorganisms are related to the deficiency of ceramide in the horny layer of the epidermis, which increases the hyper sensitivity of skin in AD patients by impairing the permeability barrier.