Time-kill kinetics and cell surface morphology of Oroxylum indicum extract against clinical strains Staphylococcus aureus and Staphylococcus intermedius

The flavone-rich Oroxylum indicum young fruit extract has been reported to effectively inhibit five clinically important zoonotic bacteria including Staphylococcus intermedius and Staphylococcus aureus, and the minimum inhibitory concentrations (MIC) were identified. Mechanisms of the antibacterial activities were suggested to be chemical–pathogen interactions involved in cell wall or cell membrane dysfunction, direct damage to the cytoplasmic membrane or indirect damage through autolysis/weakening of the cell wall and osmotic lysis. The exploration of interactions between S. intermedius, S. aureus, and O. indicum young fruit extract at 2.5 mg/mL were performed compared to amoxycillin and cefotaxime using time-kill kinetic assays over 24 hours. High magnification scanning electron microscopy (SEM) for bacterial cellular surface morphological changes at MIC concentration was completed. The prominently affected S. intermedius and S. aureus were found shortly at 3-hour (h) post incubation. The bacterial cell number showed a 54.56-fold decrease in S. intermedius and a 32.74-fold decrease in S. aureus. The viable bacteria and the bacterial growth pattern were distinctly altered after 6-h post incubation. It reached 3.6 log cfu/mL in S. intermedius and 4.2 log10 cfu/mL in S. aureus at 24-h post incubation. The bacterial particles were found smaller, less than one-half in size, and aggregated into large clumps. Irregular cellular surface with membrane morphological changes, cell membrane shrinkage, cell eruption with content leakage and cellular fusion were found. Further investigation is ongoing. This information will be beneficial for the formula development and clinical use of O. indicum to fit with the reality of the important zoonotic bacteria.

Publication History

Article published online:
16 November 2023