TiO2 nanoparticles suppress Escherichia coli cell division in the absence of UV irradiation in acidic conditions.

TiO(2) nanoparticles (NPs) activated by UV irradiation are known to have a bactericidal effect. In this study we report the details of TiO(2) NPs influence on the colony-forming capacity of E. coli in the dark at pH 4.0-4.5. At this pH the bacterial cells are negatively charged and TiO(2) NPs present a positive charge. A 60 min contact between E. coli with TiO(2) at concentrations of 0.02-0.2 mg/mL led to a reduction of E. coli cell number from 10(8) to 10(4)CFU/mL. After the reduction the system remains unchanged during the subsequent incubation. The observed reduction was a function on the initial E. coli concentration. In the presence of 0.04 mg/mL TiO(2) the colony-forming units (CFU) reduction after 60 min was of four-five orders of magnitude when the initial concentration was 10(8) cells/mL. But when starting with an E. coli concentration of 10(7) cells/mL the cell number reduction was less than one order of magnitude. Less than one order of magnitude cell number reduction was also observed for suspensions of E. coli 10(8) cells/mL and 0.002 mg/mL of TiO(2). The bacteria number reduction was always accompanied by the formation of cell aggregates. During cell incubation with TiO(2), the pH of the suspension increased, but did not reach the TiO(2) isoelectric point (IEP). E. coli cells stained with the fluorescent dye acridine orange (AO) showed that the fluorescence of single cells remained unchanged after incubation in the presence of TiO(2). The color change of fluorescence was revealed only in aggregated cells. This suggests changes in the physiologic state of E. coli incorporated into the aggregates. Aggregates of E. coli occur due to the electrostatic interaction between TiO(2) NPs and the bacterial cell surface. A hypothesis is suggested in this study to explain the CFU reduction and the retention of a certain irreducible number of cells capable of further division in the suspension in the presence of TiO(2) in the dark.