Microcalorimetric evaluation of the effects of three anthraquinone derivatives from Chinese Rhubarb and the synergistic effect of the mixture on Staphylococcus aureus

In this study, a noninvasive and nondestructive microcalorimetric method was used to investigate the antimicrobial activity of three anthraquinone derivatives (emodin, aloe-emodin and physcion) from Chinese Rhubarb. Additionally, we observed a synergistic antibacterial effect of a mixture (emodin + aloe-emodin) on Staphylococcus aureus. Antibacterial effects were further evaluated through principle component analysis and the half-inhibitory concentration (IC50) according to the influence of the anthraquinone derivatives on eight quantitative thermokinetic parameters, which were measured by isothermal microcalorimetry and obtained from metabolic power–time curves of Staphylococcus aureus growth at 37 °C. The inhibitory actions of the anthraquinone derivatives varied at different concentrations. The antibacterial effect of the derivatives on S. aureus was as follows: emodin + aloe-emodin (E + AE) > emodin (E) > aloe-emodin (AE) > physcion. Based on these results, the combined effect of emodin and aloe-emodin was stronger than that of each anthraquinone derivative alone. The combination of emodin and aloe-emodin is a promising antibacterial agent, providing a novel avenue for antibacterial materials.