The rate of human mortality from infectious diseases caused by antibiotic-resistant bacteria is a major public concern. Currently, the search for new strategies to treat infections and overcome antimicrobial resistance is a challenge. Nanoparticles (NPs) offer an alternative approach to biocide design that involves antibacterial mechanisms other than antibiotics. The attention is primarily attracted due to the large surface area, diverse physical and chemical properties, variety of precursor materials, and the ability to form hybrid structures. Of particular interest are metal and metal oxide NPs that can survive in harsh conditions without losing their colloidal stability. They provide long-term antibacterial activity and can destroy bacterial biofilms. This review summarizes and discusses recent progress in understanding the antibacterial mechanisms of metal/metal oxide NPs, highlighting three key aspects: physical interaction with the cell wall, release of metal ions, and the generation of reactive oxygen species (ROS). Unlike antibiotics with a single mode of action, NPs can simultaneously exhibit multiple mechanisms with a synergistic effect. This leads to the destruction of cell membranes, damage to DNAs and proteins, inactivation of enzymes, oxidation of cellular components, and ultimately, cell death. The activity of NPs against bacterial biofilms is also considered. In addition, this review focuses on the current state of the most widely used metal-containing NPs with a comprehensive discussion of the specific mechanisms for each type (Ag, Au, Cu/CuO, TiO, ZnO, FeO/FeO). Finally, the review discusses approaches to enhancing the biocidal efficacy of NPs through modification and composite formation, as well as the use of light and magnetic field.