Bactericidal activity of photocatalytic TiO(2) reaction: toward an understanding of its killing mechanism

Abstract

When titanium dioxide (TiO(2)) is irradiated with near-UV light, this semiconductor exhibits strong bactericidal activity. In this paper, we present the first evidence that the lipid peroxidation reaction is the underlying mechanism of death of Escherichia coli K-12 cells that are irradiated in the presence of the TiO(2) photocatalyst. Using production of malondialdehyde (MDA) as an index to assess cell membrane damage by lipid peroxidation, we observed 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 cell death. Under these conditions, concomitant losses of 77 to 93% of the cell respiratory activity were also detected, as measured by both oxygen uptake and reduction of 2,3,5-triphenyltetrazolium chloride from succinate as the electron donor. The occurrence of lipid peroxidation and the simultaneous losses of both membrane-dependent respiratory activity and cell viability depended strictly on the presence of both light and TiO(2). We concluded that TiO(2) photocatalysis promoted peroxidation of the polyunsaturated phospholipid component of the lipid membrane initially and induced major disorder in the E. coli cell membrane. Subsequently, essential functions that rely on intact cell membrane architecture, such as respiratory activity, were lost, and cell death was inevitable.

FIG. 1

Effects of light and TiO₂ on lipid peroxidation of E. coli. Cells (2.5 × 10⁸ CFU ml⁻¹) were incubated in the dark, in UV light, in the dark with TiO₂ (0.1 mg ml⁻¹), and in UV light with TiO₂ (0.1 mg ml⁻¹) for 30 min with continuous stirring. The light intensity was 8 W m⁻². MDA was quantified by the TBA assay.

FIG. 2

Kinetics of lipid peroxidation in E. coli induced by TiO₂ photocatalysis. Cell suspensions (1.8 × 10⁹ CFU ml⁻¹) were treated with TiO₂ (1 mg ml⁻¹) and UV light (8 W m⁻²) for various periods of time. MDA was quantified by the TBA assay.

FIG. 3

Kinetic losses of respiratory activity and viability of E. coli induced by TiO₂ photocatalysis. Cells (1.2 × 10⁸ CFU ml⁻¹) were treated with TiO₂ (0.5 mg ml⁻¹) and incubated under UV light (8 W m⁻²). Respiratory activity was determined by measuring the reduction of oxygen and the reduction of TTC to TTF. Viability was determined by the plate count method. Gray bars, oxygen uptake; black bars, TTC reduction; open bars, survival. The 100% activities at time zero were 16 nmol of O₂ · min⁻¹ · mg (dry weight) of cells⁻¹ for oxygen uptake and 0.27 nmol of TTF · min⁻¹ · mg (dry weight) of cells⁻¹ for TTC reduction.