To investigate the ultrastructural changes, X-ray microanalysis and leakages of protein and lipids from bacteria treated with the ethanolic extract of Acacia mearnsii. The ultrastructural changes were investigated using scanning electron microscope (SEM). The X-ray microanalyses were determined using electron dispersive spectroscopy (EDS) while the protein and lipid leakages were determined spectrophotometrically. The scanning electron microscopy indicated varied ultrastructural changes in the morphology of bacterial cells treated with the ethanol extract. The X-ray microanalysis showed significant differences between the elemental contents of extract-treated and broth-cultured and untreated bacteria strains. Lipids and proteins were leaked to a greater extent from the extract-treated bacterial strains in comparison with the broth-cultured and untreated ones used as controls. The possible antibacterial effects of the ethanol extract of A. mearnsii could be inhibition of a significant step in peptidoglycan assembly, inhibition of metabolic processes as well as the cell wall and cell membrane disruptions resulting in the efflux of lipid and protein in all the bacteria tested. This is a report identifying elemental components and associating their disintegrations in bacterial isolates with the antibacterial activities of medicinal plants. While ethanol extract of A. mearnsii caused ultrastructural changes, breakdown of elemental components into different elements and leaked lipid and protein from treated bacteria, the physiological damages to the cell wall further justify the bactericidal ability of the extract.