Effluent from wastewater treatment plants contains a wide variety of engineered nanoparticles (ENPs)released from different sources. Although single type ENPs have been studied extensively with respect to their environmental impact, ENPs in mixed forms have not been investigated much at environmentally relevant concentrations. This study was designed to test the effect of mixed ENPs at three combinations and concentrations on an aquatic bacterial community. After mixing artificial treated wastewater with river water and exposing the microbial community to ENPs for three days, the ENPs were characterized by SP-ICP-MS. Results from this study showed that: 1)the size distribution of Ti and Zn at the beginning and end of the experiment did not vary much among all tested conditions. For Ag, the most frequent size increased more than 2-fold when the highest Ag ENPs were added; 2)particle concentrations of ENPs generally correlated positively with added concentrations; 3)dissolved Zn and Ag increased significantly as a result of spike; and 4)the bacterial community structure was shifted significantly as a consequence of ENPs' addition. With the dominant population being suppressed, the community exposed to ENPs became more diverse and even. Surprisingly, further increase of the doses of the three ENPs did not bring significant change to the microbial community. These results revealed that ENPs could bring significant impacts to prokaryotes even at low concentrations. But these impacts do not necessarily correlate positively with doses.