Antibacterial and anti-quorum sensing activities of the different solvent extracts of Embelia ruminata

Abstract

The rise of antibiotic resistance has resulted in an intense drive to screen plants and their secondary metabolites for their antibacterial activity as alternatives to conventional antibiotics. Furthermore, research has focused on plant sources that can curtail the development of drug resistance and the expression of bacterial virulence factors by attenuation of quorum sensing (QS). Thus, the present study investigated the phytochemical profile together with the antibacterial potential of a southern African plant species, Embelia ruminata (E.Mey. ex A.DC.) Mez. against a panel of Gram-negative and Gram-positive strains. Additionally, the study evaluated the QS inhibitory potential of E. ruminata using Chromobacterium subtsugae CV017 (short chain autoinducer) and Chromobacterium violaceum ATCC 12472 (long chain autoinducer). The results of the preliminary phytochemical tests showed the presence of benzoquinone, alkaloids, triterpenoids, flavonoids, phenolic compounds, quinones anthraquinones and resin compounds in the different solvent extracts. In the gas chromatography-mass spectrophotometer analyses, thirty bioactive compounds were identified including the benzoquinone compound, embelin in the hexane and chloroform fruit and seed extracts. In the antibacterial assays, extracts tested at 400 and 800 µg produced limited antibacterial activity against the Gram-negative bacteria, with the fruit extracts demonstrating greatest activity against selected bacterial strains. The methanolic stem bark extracts showed promising activity against selected Gram-positive strains, particularly methicillin-resistant Staphylococcus aureus. In the anti-QS violacein inhibition assay, the chloroform leaf, hexane and methanolic seed extracts displayed inhibition of both biosensor strains suggesting that these extracts possess broad-spectrum QS inhibitory activity. Furthermore, the methanolic seed extracts demonstrated the most potent violacein inhibitory activity against both biosensor strains compared to the other plant organ extracts. This study provides insight into the bioactivity of E. ruminata extracts as possible anti-virulence agents targeting resistant bacterial strains.