The bacterial pathogen, Staphylococcus aureus, frequently colonizes patients with Atopic Dermatitis (AD), a skin disease associated with loss of barrier function (Boguniewicz and Leung, 2011). Elevated levels of staphylococcal lipoteichoic acid (LTA) are found on the skin of patients affected by severe AD (Travers et al., 2010), however, the relative contributions of this product to keratinocyte differentiation and skin barrier formation are unknown.

To identify gene expression pathways altered by staphylococcal LTA, we performed gene microarray analysis on RNA isolated from primary human keratinocytes treated with media control or with LTA. We found keratinocytes are highly responsive to this bacterial product with 302 genes being either repressed or activated at least 2-fold (Supplementary Table S1). The biological process most significantly affected by LTA was that of epidermal development (Fig. 1a) with a 14.6-fold change. Other processes affected included the response to wounding, keratinocyte proliferation, negative regulation of cell differentiation and changes in the Notch signaling pathways. We focused our studies on examination of genes involved in the keratinocyte differentiation process. Fig. 1b and Supplementary Table S2 show that staphylococcal LTA down regulated a number of genes essential for keratinocyte differentiation, including KRT-1, KRT-10, and DSC-1 (Candi et al., 2005). The Notch ligands, JAG-2 and DLL-1, were similarly repressed. Interestingly, several genes involved in cell proliferation, including ID-1, FOS and Cyclin A1 were prominently upregulated by LTA. To validate the results of the microarray, we measured expression of genes by real-time PCR, primarily focusing on genes known to play a critical role in the keratinocyte differentiation process and barrier formation. RT-PCR established that KRT-1,