Dioctadecyldimethylammonium bromide (DODAB) vesicles kill Escherichia coli in the micromolar range of DODAB concentrations. At 1.2 x 10(6) bacteria/mL, the minimum bactericidal concentration is smaller than 0.5 mu M DODAB for an interaction time of 24 h. DODAB effects on the cells are described using microelectrophoresis, viable counts and turbidimetry over a range of bacterial number densities, and DODAB concentrations. Electrophoretic mobility for the cells as a function of DODAB concentration establishes the DODAB amount required to attain charge neutralization on the cell surface and maximal flocculation rate and extent. From 10(8) bacteria/mL, there is rapid bacteria flocculation induced by DODAB. However, cell death is not related to aggregation taking place also for nonaggregated cells at much smaller bacteria number densities. The results may be of importance for improving water quality in contaminated reservoirs.