Biological pollution is one major cause of the degradation of indoor air quality. It was shown that microbial communities from outdoor might impact significantly the communities detected indoor. In addition, microbial contamination of the surfaces of building materials and their release into the indoor air also significantly affect indoor air quality. Preventing the growth or at least reducing the amounts of microorganisms growing on indoor building materials is essential for reducing health risks for building occupiers. Photoactive TiO2 has been widely studied as a photocatalyst that enable the inactivation of various bacterial strains. In this paper, we compare the antifungal activity of nanoparticles of TiO2 on Aspergillus niger spores and its antibacterial activity on Escherichia coli under low light irradiation, near to common indoor values. The antimicrobial activity of TiO2, expressed as log reduction, was assessed under UV irradiation in a sludge mixture of sterile water, suspension and nanoparticles of TiO2. The results showed a strong bactericidal activity of TiO2 on E. coli and a weak fungicidal activity against A. niger. Different parameters including concentration of TiO2, intensity of light, and duration of contact between TiO2 and microbial cells and spores, were investigated and significantly affected the antibacterial activity of TiO2 while poorly affected its antifungal activity. Results of this study confirmed previous investigations on antibacterial activity of TiO2 on E. coli and bring new insight on antifungal activity on the spores of A. niger. The effectiveness of the antimicrobial activity is enhanced by the duration of contact between suspension and TiO2 nanoparticles through the stirring experiments for 2H, 4H and 24H.