Aqueous silver nanoparticle was synthesized in a single step by a green biosynthetic method using aqueous leaf extracts of Premna herbacea, a local herb of Bodoland, Assam, India which acts as reducing as well as capping agent. Absorption spectra of synthesized nanoparticles were taken by UV spectrophotometer which showed a characteristic surface plasmon resonance (SPR) peak centred at a wavelength of 425 nm. The TEM analysis showed the particles were spherical and size between 10-30 nm. The bactericidal activity of the nanoparticles was tested against two gram negative bacteria (Shigella dysentrieae and E.coli) causing human dysentery. Antimicrobial property of the synthesized molecules was evaluated by calculating MIC values, disc diffusion assay and time kill kinetics based on nanoparticles concentration. MIC values were for Shigella dysentrieae and E.coli were 55 and 70 µg/ml respectively. The efficiency of antimicrobial activity by silver nanoparticles on the above mentioned microorganisms propose the possibility of a more cost effective antibacterial agent against dysentery causing microbes.