Synergistic antibacterial activity of Erigeron floribundus extracts combined with erythromycin against multidrug-resistant bacteria

Abstract

Background: This research assessed the in vitro antibacterial activity of Erigeron floribundus extracts against multidrug-resistant bacterial strains and explored their potential to enhance the effectiveness of standard antibiotics through synergistic combinations. Methods: Leaf, stem, and root crude extracts of E. floribundus were obtained using methanol and ethyl acetate. Major phytochemical groups (total phenols, flavonoids, tannins, saponins) were quantified. The antimicrobial activity of the extracts alone and in combination with amoxicillin, levofloxacin, and erythromycin was evaluated against laboratory strains of Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa using the broth microdilution assay. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) were determined. Synergy was quantified using the fractional inhibitory concentration (FIC) index. Results: The methanol leaf extract contained the highest levels of phenolic and flavonoid compounds and exhibited the strongest intrinsic antibacterial activity. When combined with erythromycin, this extract demonstrated potent synergy against all tested bacterial strains. Combinations with amoxicillin and levofloxacin produced additive or indifferent effects depending on the bacterial species. The observed synergy indicates that the extract potentiates erythromycin's antibacterial mechanism. Conclusion: The methanol leaf extract of E. floribundus possesses considerable antibacterial properties and, more importantly, exhibits universal synergistic interaction with erythromycin against multidrug-resistant bacteria. This finding corroborates the ethnomedical application of the plant and paves the way for the plant and highlights its potential as a source of resistance-modifying compounds. Such an approach could help "rescue" existing antibiotics and provide a new strategy against antibiotic resistance.