Metallic nanoparticles against bacteria have increased recently due to their unique properties. Many metals like silver, gold, copper, aluminum, zinc and their oxides have been shown to have antibacterial properties. The activity of the nanoparticles is affected by their physico-chemical properties. Different types of mechanisms are proposed for the antibacterial actions against various types of bacteria. The metal-based nanoparticles are synthesized by the top-down methods and bottomup methods. However, the latter methods are used effectively against many types of bacteria including antibiotic-resistant bacteria.