Baicalein Inhibits the Staphylococcus aureus Biofilm and the LuxS/AI-2 System in vitro

Staphylococcus aureus (S. aureus) is a common cause of mastitis in dairy cows, a condition that has a significant economic impact. S. aureus displays quorum sensing (QS) system-controlled virulence characteristics, like biofilm formation, that make therapy challenging. In order to effectively combat S. aureus, one potential technique is to interfere with quorum sensing.

This study evaluated the effects of different Baicalin (BAI) concentrations on the growth and the biofilm of S. aureus isolates, including the biofilm formation and mature biofilm clearance. The binding activity of BAI to LuxS was verified by molecular docking and kinetic simulations. The secondary structure of LuxS in the formulations was characterized using fluorescence quenching and Fourier transform infrared (FTIR) spectroscopy. Additionally, using fluorescence quantitative PCR, the impact of BAI on the transcript levels of the luxS and biofilm-related genes was investigated. The impact of BAI on LuxS at the level of protein expression was also confirmed by a Western blotting investigation.

According to the docking experiments, they were able to engage with the amino acid residues in LuxS and BAI through hydrogen bonding. The results of molecular dynamics simulations and the binding free energy also confirmed the stability of the complex and supported the experimental results. BAI showed weak inhibitory activity against S. aureus but significantly reduced biofilm formation and disrupted mature biofilms. BAI also downregulated luxS and biofilm-associated genes' mRNA expression. Successful binding was confirmed using fluorescence quenching and FTIR.

We thus report that BAI inhibits the S. aureus LuxS/AI-2 system for the first time, which raises the possibility that BAI could be employed as a possible antimicrobial drug to treat S. aureus strain-caused biofilms.