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 the permeability barrier of the epidermis, resulting in the characteristic dry and easily
antigen-permeable skin of AD, since
ceramide serves as the major water-holding molecule in the extracellular space of the horny layer. On the other hand, the skin of such patients is frequently colonized by bacteria, most typically by Staphylococcus aureus, possessing genes such as those for
sphingomyelinase, which are related to
sphingolipid metabolism. We therefore tried to identify a possible correlation between the
ceramide content and the bacterial flora obtained from the skin of 25 patients with AD versus that of 24 healthy subjects, using a thin-layer chromatographic assay of the
sphingomyelin-associated
enzyme activities secreted from the bacteria. The findings of the assay demonstrated that
ceramidase, which breaks
ceramide down into
sphingosine and
fatty acid, was secreted significantly more from the bacterial flora obtained from both the lesional and the nonlesional skin of patients with AD than from the skin of healthy subjects;
sphingomyelinase, which breaks
sphingomyelin down into
ceramide and
phosphorylcholine, 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 finding that the skin of patients with AD is colonized by
ceramidase-secreting bacteria thus suggests that microorganisms are related to the deficiency of
ceramide in the horny layer of the epidermis, which increases the
hypersensitivity of skin in AD patients by impairing the permeability barrier.