Antibacterial Effects of Phlomoides molucelloides (Bunge) Salmaki

The insufficiency of existing antibiotics in the combat against antibiotics-resistant bacteria has necessitated the discovery of new and effective antibacterial drugs. The discovery that bacteria synthesize various virulence factors by the quorum sensing system has suggested that quorum sensing inhibitors may be used in the fight against infectious diseases. This study aimed to determine the antibacterial and anti-quorum sensing activities of methanol and water extracts of aerial and root parts of Phlomoides molucelloides (Bunge) Salmaki. The minimal inhibitory concentration (MIC) values of the extracts were investigated against reference bacterial strains using the broth microdilution method. Anti-quorum sensing activities were examined by violacein and pyocyanin pigments inhibition and swarming motility inhibition assays against Chromobacterium violaceum ATCC 12472 and Pseudomonas aeruginosa PAO1 bioreporter strains. The antibiofilm activities of the extracts were tested against P. aeruginosa PAO1 using the crystal violet staining method. The MIC value (> 2000 µg/mL) of all extracts against the tested bacteria could not be determined at the concentrations studied. All extracts partially inhibited the swarming motility of P. aeruginosa. Methanol extract of the aerial part inhibited pyocyanin production by 81.7% without interfering with P. aeruginosa growth. The extracts had no significant inhibitory activity on biofilm formation and violacein pigment production. These results showed that the extracts of P. molucelloides may be good anti-quorum sensing agents. Further research can be done to elucidate the mechanisms underlying these biological activities.