Nanosized particles of zinc oxide have gained much attention due to several applications which includes bacterial inhibition. Therefore, this work evaluates zinc oxide properties synthesized using reduction (chemical) and bio-reduction (green) processes and their corresponding inhibition potentials. The bio-reduction process was achieved using Amaranthus spinosus at 70C while the reduction process was initiated in the chemical process using sodium hydroxide. The optical measurement of ZnO was carried out using an UV–Vis spectrophotometer. The structural and morphological properties of the synthesized ZnO were evaluated using Fourier transform infrared spectroscopy, X-ray diffractometry and scanning electron microscopy. The elemental composition was carried out using energy dispersive X-ray spectroscopy. The antimicrobial activity property of the nanoparticles was tested against Pseudomonas aeruginosa, Salmonella typhi and Shigella dysenteriae. The degree of susceptibility of ZnO nanoparticles was higher in the bio-reduction process than chemically synthesized for selected microorganisms. A sustainable pathway for development of bio-antibiotic is presented.