Metal oxide nanoparticles are known to possess strong antimicrobial properties. Titanium dioxide (titania) nanoparticles have wide range antimicrobial as well biomedical applications. In the present work the antimicrobial properties of Titanium dioxide-anatase nanoparticles were investigated in the concentration range 10-100 ppm using Escherichia coli, Bacillus subtilis and Pseudomonas aeruginosa. The average size of the nanoparticles used was measured to be below 25 nm. The concentration dependent growth inhibitory effect of titania nanoparticles on the three bacterial strains were compared based on batch growth kinetic data (by dynamic growth rate). Bacterial sensitivity to nanoparticles was found to vary depending on the microbial species. The mode of antibacterial action was investigated by assaying extracellular proteins, the integrity of the cell membranes and the permeability of the inner membrane (IM) of the microbial strains. The results suggested that titania caused the antibacterial effect through membrane damage mechanisms. FT-IR studies revealed that most of amide-I, amide -II groups were absent in the nanoparticles treated cells when compared with that of control, corroborating the cell damage mode of action. The antimicrobial effects of the nanoparticles essentially differed between the microbial strains due to structural differences in the cell membranes. © 2010 Bentham Science Publishers Ltd.