Novel Insights into the Antibacterial Potential of Euphrasia officinalis L.

Regarding the resistance of bacteria to conventional antibiotics, the search for novel natural antibacterial agents is of significant importance. Herein, the antibacterial activity of ethanol and acetone extracts of the medicinal plant Euphrasia officinalis was evaluated. For the first time, the optimization of ultrasound-assisted extraction conditions to achieve a higher concentration of polyphenols and increase the antibacterial activity of E. officinalis extracts was performed using response surface methodology. Polyphenolic compounds were quantified through spectrophotometric and chromatographic techniques. The optimized extracts' antibacterial activity was analyzed via broth microdilution, time-kill, and cell membrane permeability assays. The optimized extracts exhibited strain-dependent antibacterial activity. Gram-positive bacteria (Staphylococcus aureus LM11, S. aureus LM12, and Enterococcus sp. LM5) were the most susceptible, at an extract concentration of 10 mg/mL. Tested extracts demonstrated a bactericidal effect after 24 h. Additionally, the extracts impact the cell membrane's permeability, disrupting its normal function which is the first report on the potential mode of action of tested E. officinalis extracts. Major compounds identified in optimized extracts were rutin (9.52 mg/kg dry weight (DW), ellagic acid (7.1 mg/kg DW), quercetin 3-O-glucoside (4.78 mg/kg DW), and isorhamnetin 3-O-rutinoside (3.21 mg/kg DW). This study provides a comprehensive evaluation of the extraction process, phytochemical composition, and antibacterial properties of E. officinalis extracts, laying the basis for developing new applications of these extracts as natural antibacterial agents.