Aggregation of Staphylococcus aureus following treatment with the antibacterial flavonol galangin: Aggregation of S. aureus by galangin

Aim: 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 an aggregatory effect upon bacterial cells. Methods and results: In preparatory time-kill assays, 50 μg ml-1 galangin was found to reduce colony counts of ~5 x 107 cfu ml-1 Staphylococcus aureus NCTC 6571 by approximately 15,000-fold during 60 min incubation. Subsequent light microscopy studies demonstrated significant increases in the number of large clusters of bacterial cells in populations treated with the flavonol. Conclusion: Data presented here shows that galangin causes aggregation of bacterial cells. Significance and impact of study: 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 MBC assays in previous investigations were at least partially attributable to this aggregatory effect. The possibility exits 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.