The bactericidal activity of copper-deposited titanium dioxide thin film (Cu/TiO{sub}2) was investigated under very weak ultraviolet (UV) light illumination. To elucidate the roles of the film photocatalyst and the deposited copper in the bactericidal activity, cellsfrom a copper-resistant Escherichia coli (E. coli} strain were utilized. A decrease in survival rate was not observed with the copper-resistant cells under dark conditions, but when illuminated with a very weak UV intensity of 1μW/cm{sup}2, the survival rate decreased, suggesting photocatalytic bactericidal activity. The decay curve of survival on the Cu/TiO{sub}2 film under very weak UV light illumination consisted of two steps, similar to the survival change of normal E. coli on TiO{sub}2 films under rather strong UV illumination. The first step is due to the partial decomposition of the outer membrane in the cell envelope by a photocatalytic process, followed by permeation of the copper ions into the cytoplasmic membrane. The second step is due to a disorder of the cytoplasmic membrane caused by the copper ions, which results in a loss of the cell's integrity. These processes explain why the Cu/TiO{sub}2 film system shows an effective bactericidal activity even under very weak UV light illumination.
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