In this study, the possible mechanisms of action of acetone, methanol and aqueous extracts of Acacia mearnsii were investigated. The influences of these extracts on the ultrastructures, protein and lipid leakages of five different bacteria were determined. The results showed that the different extracts had varied effects on the different isolates treated while the untreated isolates remained intact. On the extract-treated isolates, the extracts caused significant ultrastructural changes, protein and lipid leakages. There were disruptions in the outer wall and cytoplasmic membranes, especially, at the polar regions of the cells, whole cell collapse and presence of extruded cellular materials situated close to the collapsed end of the cytoplasm. The morphological changes and the observed leakages showed significant antibacterial effect and membrane disruptions resulting in leakages and efflux of disintegrated cellular materials. The distinct morphological changes such as cell elongation and roughening of the surfaces suggested that the extract may have interfered with bacterial cell wall synthesis leading to cell deformations. While aqueous extract was the most effective in causing protein leakages, methanol extract was the leading cause of lipid leakages. The leakages were time and concentration dependents and, in some instances, significantly different from extract to extract. The possible mechanism of action involved in the lipid and protein leakages in the bacterial cells could be attributed to lipid peroxidation and protein oxidation owing to the antioxidants activities of the extracts being beyond protective levels. The study showed that the different extracts of A. mearnsii had bactericidal effects against the test isolates, caused ultrastructural changes and leakages resulting from the disruption of the cytoplasmic membranes of the bacterial populations.