Aggregation of Staphylococcus aureus following treatment with the antibacterial flavonol galangin

• Published in: Journal of applied microbiology 2007-11, Vol.103 (5), p.1562-1567
• Main Authors: Cushnie, T.P.T, Hamilton, V.E.S, Chapman, D.G, Taylor, P.W, Lamb, A.J
• Format: Article
• Language: English
• Subjects: aggregation; antibacterial; Bacterial Adhesion - drug effects; bactericidal; bacteriostatic; Biological and medical sciences; Colony Count, Microbial; flavonoid; Flavonoids - analysis; Flavonoids - pharmacology; Flavonols - pharmacology; Fundamental and applied biological sciences. Psychology; galangin; Helichrysum; Helichrysum - chemistry; MBC; Microbiology; Plant Extracts; Propolis; Propolis - chemistry; Staphylococcus aureus; Staphylococcus aureus - drug effects; Staphylococcus aureus - physiology; time-kill
• ISSN: 1364-5072; 1365-2672
• DOI: 10.1111/j.1365-2672.2007.03393.x

The flavonol galangin, an antimicrobial constituent of the traditional medicines propolis and Helichrysum aureonitens, is being assessed as part of an ongoing investigation into the antibacterial activity of flavonoids. The present study sought to establish whether galangin has any aggregatory effect on bacterial cells. In preparatory time-kill assays, 50 μg ml⁻¹ of galangin was found to reduce colony counts of c. 5 x 10⁷ CFU ml⁻¹Staphylococcus aureus NCTC 6571 by approximately 15 000-fold during 60 min of incubation. Subsequent light microscopy studies demonstrated significant increases in the number of large clusters of bacterial cells in populations treated with the flavonol. Data presented here show that galangin causes aggregation of bacterial cells. The finding that galangin causes bacterial cells to clump together may implicate the cytoplasmic membrane as a target site for this compound's activity. More importantly, this observation indicates that decreases in CFU numbers detected in time-kill and minimum bactericidal concentration (MBC) assays in previous investigations were at least partially attributable to this aggregatory effect. This raises the possibility that galangin is not genuinely bactericidal in action, and calls into question the suitability of time-kill and MBC assays for determining the nature of activity of naturally occurring flavonoids.