The antibacterial efficiency of longwave UV-irradiated TiO2 thin films as well as the ultrastructural damage on bacterial cells was evaluated using Pseudomonas aeruginosa as a model. The quantitative antibacterial efficiency assays showed a bacterial inhibition in the range of 32–72% at different times of irradiation. Transmission electron microscopy (TEM) was used to detect the effect of irradiation of TiO2 thin films on the ultrastructure of the bacterial cell in order to reveal possible cellular damage. After 40 min irradiation, an abnormal cellular division was observed: instead of a normal septum, an ‘elongated bridge’ was formed. At a longer irradiation time, wavy structures all around the outer cell membrane were observed, and also some bubble-like protuberances, which expelled inner material. The mechanism of irreversible bacterial cell damage caused by the photocatalytic effect of TiO2 could be related to abnormal cell division, aside from the reported physicochemical alteration of the cell membrane.