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Background  Patients with papulopustular rosacea have a higher density of Demodex folliculorum mites on their faces than normal subjects but the role, if any, of their mites in initiating inflammation is disputed. Selective antibiotics are effective in reducing the inflammatory changes of papulopustular rosacea, but their mode of action is unknown. Objectives  To investigate whether a D. folliculorum-related bacterium was capable of expressing antigens that could stimulate an inflammatory immune response in patients with rosacea. Methods  A bacterium ( Bacillus oleronius) was isolated from a D. folliculorum mite extracted from the face of a patient with papulopustular rosacea, and was investigated further. Results  This bacterium produced antigens capable of stimulating peripheral blood mononuclear cells proliferation in 16 of 22 (73%) patients with rosacea but only five of 17 (29%) control subjects ( P = 0·0105). This antigenic preparation was fractionated into 70 subfractions and the proteins in each fraction were visualized by sodium dodecyl sulphate–polyacrylamide gel electrophoresis. Western blot analysis revealed the presence of two antigenic proteins of size 62 and 83 kDa in fractions when probing with sera from patients with rosacea. No immunoreactivity to these proteins was recorded when probing with sera from control patients. Two-dimensional electrophoretic separation was used to isolate these proteins and matrix-assisted laser desorption/ionization time-of-flight analysis was employed to identify the relevant peptides. The 62-kDa immunoreactive protein shared amino acid sequence homology with an enzyme involved in carbohydrate metabolism and signal transduction while the 83-kDa protein was similar to bacterial heat shock proteins. Conclusions  Antigenic proteins related to a bacterium ( B. oleronius), isolated from a D. folliculorum mite, have the potential to stimulate an inflammatory response in patients with papulopustular rosacea. [ABSTRACT FROM AUTHOR]