Investigation of the antibacterial activity of 3-O-octanoyl-(-)-epicatechin.

AIMS: To measure antibacterial activity of the semi-synthetic flavonoid 3-O-octanoyl-(-)-epicatechin and investigate the mechanism of action. METHODS AND RESULTS: MICs determined by the broth microdilution method were 50 microg ml(-1) for beta-lactam sensitive and resistant Staphylococcus aureus, and 100 microg ml(-1) for vancomycin sensitive and resistant enterococci. In time-kill studies, 100 microg ml(-1) 3-O-octanoyl-(-)-epicatechin reduced colony forming unit numbers of antibiotic sensitive and methicillin-resistant Staph. aureus below detectable levels within 120 min. Bacterial aggregation was not observed when cells exposed to 3-O-octanoyl-(-)-epicatechin were examined by light microscopy. It was also shown that 50 microg ml(-1) 3-O-octanoyl-(-)-epicatechin is capable of reducing colony forming unit numbers of high cell density Staph. aureus populations by 80-fold within 60 min incubation, and inducing leakage of 50% of their internal potassium within just 10 min. CONCLUSIONS: 3-O-Octanoyl-(-)-epicatechin is active against Gram-positive bacteria, has bactericidal activity against both antibiotic sensitive and resistant strains, and is likely to exert its primary antibacterial effect by damaging the cytoplasmic membrane. SIGNIFICANCE AND IMPACT OF THE STUDY: 3-O-Octanoyl-(-)-epicatechin has significant antibacterial activity and additional structural modification and/or formulation studies may allow this to be potentiated.

Aggregation of Staphylococcus aureus following treatment with the antibacterial flavonol 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 any aggregatory effect on bacterial cells. METHODS AND RESULTS: In preparatory time-kill assays, 50 microg ml(-1) of galangin was found to reduce colony counts of c. 5 x 10(7) CFU ml(-1)Staphylococcus 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.

Assessment of the antibacterial activity of galangin against 4-quinolone resistant strains of Staphylococcus aureus.

The flavonol galangin is present in numerous plants and is a major constituent of Helichrysum aureonitens, a perennial herb used by South African indigenes to treat infection. In the present study, the antibacterial activity of galangin was assessed against 17 strains of 4-quinolone resistant S. aureus using an agar dilution assay. It was determined that the flavonol had a minimum inhibitory concentration (MIC) of approximately 50 microg/ml against 16 of these strains, including those which exhibited 250- and 500-fold increases in norfloxacin resistance. The remaining one strain, which possessed an amino acid alteration in the GrlB subunit of topoisomerase IV, had increased susceptibility to galangin. Control strains of 4-quinolone sensitive S. aureus were also found to have MICs of 50 microg/ml. The topoisomerase IV enzyme may therefore be implicated in the antibacterial mechanism of action of galangin. Clearly however, there is no cross-resistance between galangin and the 4-quinolones, and the flavonol therefore warrants further investigation as an antibacterial agent.