PURPOSE. A role for a bacterium, Bacillus oleronius, originally isolated from a Demodex mite, in the induction of ocular rosacea has been proposed. The aim of this work was to characterize the response of a corneal epithelial cell line to Bacillus proteins, as this might give an insight into how such proteins contribute to the symptoms of ocular rosacea in vivo. METHODS. The effect of exposing Bacillus protein preparation on human telomeraseimmortalized corneal epithelial cells (hTCEpi) was measured by monitoring changes in cell proliferation and the expression of a number of genes associated with inflammation. The production of inflammatory cytokines was measured and the expression and activity of MMP- 9 was quantified. RESULTS. Exposure of hTCEpi cells to 2 or 6 lg/mL Bacillus protein resulted in a dosedependent reduction in cell proliferation. Exposure of cells to 6 lg/mL Bacillus protein did not induce apoptosis, but there was an increase in the expression of genes coding for IL-6 (13.8-fold), IL-1b (4.0-fold), IL-8 (11.1-fold), and TNF-a (4.1-fold). Increased expression of genes coding for the defensins, CCL20 (4.5-fold) and S100A7 (6.8-fold) also was observed. Elevated production of IL-6 and IL-8 was evident from cells exposed to 2 and 6 lg/mL Bacillus protein. The hTCEpi cells demonstrated increased MMP-9 expression (3.2-fold, P ¼ 0.003) and activity (2.2-fold, P ¼ 0.0186) after 48 hours of exposure to 6 lg/mL Bacillus protein preparation. CONCLUSIONS. The results suggest that interaction of Demodex-associated Bacillus proteins with the corneal surface could lead to tissue degradation and inflammation, possibly leading to corneal scarring.