Overuse and abuse of antibiotics greatly hasten the development of microbial drug resistance and substantially threat to global public health. Developing alternative methods for combating bacterial infections is urgently required. In this work, a simple hydrothermal approach was employed to prepare the protoporphyrin IX-polyethylenimine nanoparticles (PPIX-PEI NPs) containing abundant amine groups and PPIX moieties. The as-obtained PPIX-PEI NPs exhibit antibacterial properties against both Gram-positive and Gram-negative bacteria. The presence of PPIX in the PPIX-PEI NPs can generate reactive oxygen species (ROS) under 635 nm laser irradiation, which enhance the antibacterial properties of the PPIX-PEI NPs against Gram-positive bacteria. Thus, the PPIX-PEI NPs display a synergistic antibacterial activity against Gram-positive bacteria in the combination of antibacterial photodynamic therapy (PDT). In addition, emission of red fluorescence by the PPIX-PEI NPs can help to differentiate bacteria and observe the bacterial morphologies using a confocal laser scanning microscope (CLSM). (C) 2019 Elsevier Inc. All rights reserved.