Metal nano particles have an enormous potential with several applications in which they involve with great selectivity and high efficiency. In this context, titanium dioxide nanoparticles belong to a new generation of such particles, due to their brilliant and interesting optical, dielectric, and photo-catalytic characteristics. Hence they are now being widely used in electronic devices, cosmetics, pharmaceutics and other products. Extensive usage and applications has also lead to the indiscriminate dumping of their related products, after use, into the environment. Incremental amounts may also add up in the environment as “by products” and wastage during the synthesis of their commercial products. In this study, the characterization of commercially available titanium dioxide nanoparticles has been carried out by X-ray diffraction analysis and UV-VIS spectroscopy. Their toxicity with respect to certain bacteria, both MTCC cultures and also those isolated from environmental samples has been studied by suspending these particles in water and in acidic solution.