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.
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