Synergistic antibacterial activity of ZnO nanocolloid and leaf extract of <i>Bersama abyssinica</i> against gram positive and gram-negative bacteria

Abstract The increasing prevalence of antimicrobial-resistant bacteria underscores the need for alternative antibacterial agents. This study evaluated the antibacterial activity of zinc oxide (ZnO) nanocolloids in combination with Bersama abyssinica leaf extract. The leaf extract was prepared by aqueous maceration, while ZnO nanocolloids were chemically synthesized using zinc acetate as a precursor. Characterization confirmed successful synthesis: Ultraviolet visible spectroscopy showed absorption peaks at 342 nm (ZnO), 366 nm (extract), and 371 nm (combination); x-ray diffraction revealed a wurtzite structure with an average crystallite size of 10.20 nm; and Fourier transform infrared spectroscopy confirmed the presence of hydroxyl and carbonyl groups in both the extract and nanocolloid. Antibacterial activity was evaluated against Enterococcus faecalis , Staphylococcus aureus, Pseudomonas aeruginosa , and Escherichia coli using the agar well diffusion method. The ZnO–extract combination produced inhibition zones of 14.41 ± 1.56 mm, 16.44 ± 0.41 mm, 16.53 ± 2.06 mm, and 7.63 ± 1.05 mm against Enterococcus faecalis , Staphylococcus aureus, Pseudomonas aeruginosa , and Escherichia coli , respectively, with P. aeruginosa showing the highest susceptibility. However, the strongest relative synergistic effect (140.2%) was observed against E. faecalis . The minimum inhibitory concentration values were 31.75 mg ml −1 ( S. aureus ) and 125 mg ml −1 ( P. aeruginosa ), while minimum bactericidal concentration values were 62.5 mg ml −1 ( S. aureus ) and 250 mg ml −1 ( P. aeruginosa ) but for E. faecalis and E. coli beyond 500 mg ml −1 . Overall, the combination demonstrated enhanced antibacterial efficacy compared to the plant extract alone, particularly against E. faecalis , highlighting the synergistic effects of ZnO nanocolloid and phytochemicals in extending the antibacterial spectrum. These findings suggest that ZnO– B. abyssinica nanocomposites are promising antibacterial candidates, though further studies are needed to evaluate cytotoxicity and in vivo performance.