Bactericidal activity of photocatalytic TiO2 reaction: Toward an understanding of its killing mechanism

When titanium dioxide (TiO2) is irradiated with near-UV light, this semicon ductor exhibits strong bactericidal activity. In this paper, we present the first evidence that the lipid peroxidation reaction is the underlying mech anism of death of Escherichia call K-12 cells that are irradiated in the pr esence of the TiO2 photocatalyst, Using production of malondialdehyde (MDA) as an index to assess cell membrane damage by lipid peroxidation, we obser ved that there was an exponential increase in the, production of MDA, whose concentration reached 1.1 to 2.4 nmol - mg (dry weight) of cells(-1) after 30 min of illumination, and that the kinetics of this process paralleled c ell death. Under these conditions, concomitant losses of 77 to 93% of the c ell respiratory activity were also detected, as measured by both oxygen upt ake and reduction of 2,3,5-triphenyltetrazolium chloride from succinate as the electron donor. The occurrence of lipid peroxidation and the simultaneo us losses of both membrane-dependent respiratory activity and cell viabilit y depended strictly on the presence of both light and TiO2. We concluded th at TiO2 photocatalysis promoted peroxidation of the polyunsaturated phospho lipid component of the lipid membrane initially and induced major disorder in the E. call cell membrane, Subsequently, essential functions that rely o n intact cell membrane architecture, such as respiratory activity, were los t, and cell death was inevitable.