This paper demonstrated the relative bactericidal activity of photoirradiated (6W-UV Torch, λ > 340 nm and intensity = 0.64 mW/cm) P25-TiO nanoparticles, nanorods, and nanotubes for the killing of Gram-negative bacterium Agrobacterium tumefaciens LBA4404 for the first time. TiO nanorod (anatase) with length of 70-100 nm and diameter of 10-12 nm, and TiO nanotube with length of 90-110 nm and diameter of 9-11 nm were prepared from P-25 Degussa TiO (size, 30-50 nm) by hydrothermal method and compared their biocidal activity both in aqueous slurry and thin films. The mode of bacterial cell decomposition was analyzed through transmission electron microscopy (TEM), Fourier transform-infrared (FT-IR), and K ion leakage. The antimicrobial activity of photoirradiated TiO of different shapes was found to be in the order P25-TiO > nanorod > nanotube which is reverse to their specific surface area as 54 < 79 < 176 m g, evidencing that the highest activity of P25-TiO nanoparticles is not due to surface area as their crystal structure and surface morphology are entirely different. TiO thin films always exhibited less photoactivity as compared to its aqueous suspension under similar conditions of cell viability test. The changes in the bacterial surface morphology by UV-irradiated P25-TiO nanoparticles was examined by TEM, oxidative degradation of cell components such as proteins, carbohydrates, phospholipids, nucleic acids by FT-IR spectral analysis, and K ion leakage (2.5 ppm as compared to 0.4 ppm for control culture) as a measure of loss in cell membrane permeability.