The effects of Streptococcus thermophilus on ceramide levels either in vitro on cultured human keratinocytes or in vivo on stratum corneum, have been investigated. In vitro, Streptococcus thermophilus enhanced the levels of ceramides in keratinocytes in a timedependent way. The presence of high levels of neutral, glutathione-sensitive, sphingomyelinase in Streptococcus thermophilus could be responsible for the observed ceramide increase. The application of a base cream containing sonicated

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... tococcus thermophilus in the forearm skin of 17 healthy volunteers for 7 d also led to a significant and relevant increase of skin ceramide amounts, which could be due to the T he normal morphology of stratum corneum is essential for maintaining the water barrier of the skin (Bowser and White, 1985; Rawlings et al, 1994) . The proteinenriched corneocytes are embedded in an intercellular lipid matrix which is composed primarily of ceramides (43-46% of total lipids), cholesterol, and fatty acids together with smaller amounts of cholesterol sulfate, glucosylceramides, and phospholipids (Yardley and Summerly, 1981; Elias et al, 1988) . These lipids form multilamellar sheets within the intercellular spaces of the stratum corneum, the organization of which is essential in maintaining the functionality of the skin as an effective barrier to water loss (Rougier et al, 1983; Eckert and Rorke, 1989; Potts and Francoeur, 1991) . In addition, they also play an important part in determining the mechanical, cohesive, and desquamatory properties of the stratum corneum, and therefore have a key role to counteract the environmental challenges which can lead to disturbances in skin function (for a review see Rawlings et al, 1994) . A global decrease in lipid content leads to alterations in lamellar bilayer morphology which appear to underlie the impaired functional abnormalities commonly associated with aging. Indeed, an agerelated decline in stratum corneum ceramide, cholesterol, and fatty acid levels has been previously reported (Ghadially et al, 1995; Rogers et al, 1996) . Overall, the total lipid levels decrease by Manuscript