Aggregation of Staphylococcus aureus following treatment with the antibacterial flavonol galangin

Abstract

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 any aggregatory effect on bacterial cells. Methods and Results: In preparatory time‐kill assays, 50 μg ml−1 of galangin was found to reduce colony counts of c. 5 × 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. Conclusion: Data presented here show that galangin causes aggregation of bacterial cells. Significance and Impact of the 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 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.

Recent advances in understanding the antibacterial properties of flavonoids

“…Such methods have frequently been used to test flavonoids and on many occasions bactericidal activity was reportedly detected [30,32,59]. Studies with model membranes indicate that flavonoids cause aggregation though [60,61], and in 2007 it was confirmed that the flavonol galangin has this effect on bacterial cells [62]. The flavan-3-ol epicatechin gallate has also been reported to cause bacterial aggregation [19], although it is not yet clear whether this observation was due to genuine aggregation (cells clumping together) or pseudomulticellular bacteria (cells failing to separate following binary fission [54]).…”

Propolis

“…C'est la galangine qui semble donner l'effet antistaphylococcique. Elle agirait en empêchant l'adhésion de ce germe [13]. La propolis brésilienne (ou propolis rouge) est antimicrobienne, en particulier sur S. aureus ATCC 25923 et S. mutans [2].…”

Adaptation of Salmonella enterica Serovar Senftenberg to Linalool and Its Association with Antibiotic Resistance and Environmental Persistence

“…An aggregation of bacterial cells was previously shown with other compounds that originated from plants, such as the flavonoids galangin and catechin (60). An aggregation of Salmonella on leaves contributes not only to the persistence of the pathogen on the plant but also to its resistance to disinfection approaches (61).…”