Particle size-dependent antibacterial activity of emodin nanocrystals against Staphylococcus aureus

Emodin is a natural anthraquinone derivative with poor water solubility, which limits its antibacterial activity. The purpose of this work is to investigate the antibacterial activity of emodin nanocrystals (EMD-NCs) with different particle sizes against Staphylococcus aureus (S. aureus) and explores its underlying mechanisms. EMD-NCs were prepared by wet media milling and characterized using multiple techniques. Their antibacterial activity was assessed by determining the minimum inhibitory concentration (MIC), bacterial growth curves and time-kill curves. To elucidate the mechanism of action, cell morphology, membrane permeability, membrane integrity and intracellular levels of reactive oxygen species (ROS) were examined. The dissolution of EMD-NCs and their adhesion to S. aureus cell surfaces were also measured to investigate the particle size-dependent antibacterial activity. Results showed that EMD-NCs were prepared successfully with various particle sizes. Nanocrystals enhanced the antibacterial activity of emodin significantly. After the treatment with EMD-NCs, S. aureus exhibited compromised cell membrane, outflow of macromolecules and accumulation of intracellular ROS levels. This study revealed a particle size-dependent antibacterial activity of EMD-NCs, with smaller emodin particles presenting stronger bacterial growth inhibition, which can be attributed to increased dissolution of emodin when the particle size decreased from 20.7 μm to 611 nm, and to the particle drifting effect for EMD-NCs with particle size of 611, 419 and 212 nm.