TiO2 nanoparticles (NPs) activated by UV irradiation are known to have a bactericidal effect. In this study we report the details of TiO2 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 TiO2 NPs present a positive charge. A 60 min contact between E. coli with TiO2 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 TiO2 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. coil 10(8) cells/mL and 0.002 mg/mL of TiO2. The bacteria number reduction was always accompanied by the formation of cell aggregates. During cell incubation with TiO2, the pH of the suspension increased, but did not reach the TiO2 isoelectric point (IEP). E. coil cells stained with the fluorescent dye acridine orange (AO) showed that the fluorescence of single cells remained unchanged after incubation in the presence of TiO2. 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. coil occur due to the electrostatic interaction between TiO2 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 TiO2 in the dark. (c) 2012 Elsevier B.V. All rights reserved.