Bactericidal mode of titanium dioxide photocatalysis

When exposed to near-UV light, titanium dioxide (TiO2) exhibits a strong ba ctericidal activity. However, the killing mechanism(s) underlying the TiO2 photocatalytic reaction is not yet well understood. The aim of the present study is to investigate the cellular damage sites and their contribution to cell death. A sensitive approach using o-nitrophenol beta-D-galactopyranos ideside (ONPG) as the probe and Escherichia coli as model cells has been de veloped. This approach is used to illustrate damages to both the cell envel ope and intracellular components caused by TiO2 photocatalytic reaction. Tr eatment of E. coli with TiO2 and near-UV light resulted in an immediate inc rease in permeability to small molecules such as ONPG, and the leakage of l arge molecules such as beta-D-galactosidase after 20 min. Kinetic data show ed that cell wall damage took place in less than 20 min, followed by a prog ressive damage of cytoplasmic membrane and intracellular components. The re sults from the ONPG assay correlated well with the loss of cell viability. Cell wall damage followed by cytoplasmic membrane damage leading to a direc t intracellular attack has therefore been proposed as the sequence of event s when microorganisms undergo TiO2 photocatalytic attack. It has been found that smaller TiO2 particles cause quicker intracellular damage. Evidence h as been obtained that indicated that the TiO2 photocatalytic reaction resul ts in continued bactericidal activity after the UV illumination terminates. (C) 2000 Elsevier Science S.A. All rights reserved.