The antimicrobial properties of dextrose encapsulated gold nanoparticles (dGNPs) with average diameters of 25 nm, 60 nm, and 120 nm (± 5 nm) synthesized by green chemistry principles were investigated against both Gram-negative and Gram-positive bacteria. Studies were performed involving the effect of the dGNPs on the growth, morphology and the ultrastructural properties of bacteria. dGNPs were found to have significant dose dependent antibacterial activity which was directly proportional to their size and also their concentration. The microbial assays revealed the dGNPs to be bacteriostatic as well as bactericidal. The dGNPs exhibited their bactericidal action through the disruption of the bacterial cell membrane causing leakage of cytoplasmic content. The overall outcomes of this study suggest that dGNPs hold promise as a potent antimicrobial agent against a wide range of disease causing bacteria and can control and prevent possible infections or diseases.