Sanggenon C - A novel anti-enterococcal agent from Morus alba root bark

The root bark of Morus alba L., also known as sāng bái pí, has been used for centuries in traditional Chinese medicine to treat colds and respiratory infections. Previous studies have demonstrated various antibacterial properties for M. alba, highlighting its potential as a rich source of broad-spectrum antibacterial compounds.

This study aimed to identify the antibacterial activity of M. alba root bark and its bioactive constituents against selected bacteria from the ESKAPE panel, including various strains of Enterococcus spp., Staphylococcus aureus, Klebsiella pneumoniae, and Acinetobacter baumannii.

A hydroethanolic M. alba root bark extract (MACEW60) was prepared and fractionated into 16 microfractions (MFs) by high-performance countercurrent chromatography (HPCCC). A molecular network (MN) was generated to visualize the chemical profile over the course of the MFs. The antibacterial activity of the MFs was evaluated using the broth microdilution assay, with results expressed as the percentage of bacterial growth inhibition. For the most promising MFs and constituents, the minimum inhibitory concentration (MIC) was determined.

The hydroethanolic extract MACEW60 exhibited antibacterial activities against the gram-positive bacteria Enterococcus spp. and S. aureus. Through biological testing and phytochemical analysis of the generated MFs, the mulberry Diels-Alder-type adduct (MDAA) sanggenon C was identified as the most potent antibacterial compound targeting E. faecalis, E. faecium, and S. aureus. Molecular networking revealed the occurrence of additional MDAAs, including sanggenons D and O which are diastereomers of sanggenon C. All three investigated MDAAs exhibited newly described anti-enterococcal activity but differ in potency. Sanggenon C demonstrated MIC values of 3.125 μM, followed by sanggenon D (12.5-25 μM) and O (25-50 μM), respectively.

The extract of M. alba root bark exhibited distinct antibacterial activity against gram-positive bacteria from the ESKAPE panel. Notably, this study is the first to report its anti-enterococcal activity. Phytochemical and biochemometric analyses identified sanggenonC as the primary antibacterial compound, with a favorable stereochemistry, compared to its isomers. Given their potent activity, MDAAs like sanggenonC represent a promising structural class with a unique scaffold for the development of novel antibacterial agents targeting multiple species of gram-positive pathogens.