{"id":{"repo_id":"rgu","oai_identifier":"oai:rgu-repository.worktribe.com:2807349"},"canonical_url":"https://search.dev.ndltd.org/etd/rgu/oai:rgu-repository.worktribe.com:2807349","repository":{"repo_id":"rgu","name":"Robert Gordon University","base_url":"https://rgu-repository.worktribe.com/oaiprovider"},"display":{"title":"Investigation of the antibacterial activity of selected flavonoids.","abstract":"Many bacteria have become resistant to existing drugs and there is now an urgent need for new antibacterial agents. Flavonoids are a class of natural product with a diverse range of pharmacological properties, and previous studies at The Robert Gordon University and University of London have identified several candidates suitable for antibacterial analysis. The purpose of the present research programme was to determine the range of bacteria against which these agents are effective, to identify suitable conditions for in vitro analysis of the most active compounds, to try and characterise flavonoid activity as either bacteriostatic or bactericidal, and to initiate an investigation into the underlying antibacterial mechanism(s) of action of these compounds. In the initial phase of the project, the agents polyphenon E (a green tea extract), galangin, apigenin and baicalin (naturally occurring flavonoids), and 3-O-octanoyl-(-)-epicatechin (a semi-synthetic flavonoid), were screened for inhibitory activity against antibiotic sensitive and resistant strains of Staphylococcus aureus, Enterococcus faecalis, Enterococcus faecium, Escherichia coli and Pseudomonas aeruginosa. Polyphenon E was found to have MIC values of 200 mu g/mL against S. aureus strains, and galangin was found to have MIC values of 50 mu g/mL or less against S. aureus strains, when tested using the broth microdilution and agar dilution methods respectively. MIC values of approximately 50 mu g/mL were determined for 3-O-oclanoyl-(-)- epicatechin against strains of S aureus, and MICs of 100 mu g/mL were determined for this flavonoid against strains of E. faecalis and E. faecium using the broth microdilution method. Little or no activity was detected from the remaining flavonoids when tested at concentrations up to and including 200 mu g/mL. The structural stability of antibacterial flavonoids in solution was then investigated by scanning spectrophotometry. When galangin was dissolved in aqueous sodium carbonate and incubated for 24 hours at 37°C, shifts in wavelength maxima were detected and the flavonoid’s absorbance at these values decreased to as little as 2% of their original value. The alteration in galangin’s structure, implied from these changes, was accompanied by loss of antibacterial activity. Spectrophotometric analysis indicated structural changes when galangin was incubated in aqueous ammonia too. By contrast, galangin’s wavelength maxima did not change during incubation in buffered or unbuffered aqueous DMSO and BSA solution. The wavelength maxima of 3-O-octanoyl-(-)-epicatechin also remained steady when incubated in aqueous DMSO or aqueous ethanol. Decreases in absorbance at wavelength maxima were noted when galangin and 3-O-octanoyl-(-)-epicatechin were incubated in these aqueous organic solutions, but the decreases were small and probably attributable to poor water solubility rather than poor structural stability. When time-kill experiments were performed, 50 mu g/mL galangin was found to cause decreases of 1000-fold and more in colony forming unit numbers of antibiotic sensitive S. aureus and methicillin resistant S. aureus within 60 and 120 minutes incubation respectively. Bacterial re-growth was noted after 3 to 6 hours however. At a concentration of 100 mu g/mL, 3-O-octanoyl-(-)-epicatechin reduced colony counts of both strains by more than 1000-fold in just 60 minutes. Counts were reduced below minimum detectable levels (~50 cfu/mL) within a further 60 minutes, and remained below minimum detectable levels for the duration of the 7 day experiments. In subsequent light microscopy studies, performed using the above results and results from additional time-kill assays as a guide, galangin was shown to cause aggregation of bacterial cells. These results indicate that galangin-induced decreases in colony counts of S. aureus observed in time-kill assays are caused by cell aggregation or a combination of cell aggregation and cell death, and call into question the validity of claims from past investigations with natural flavonoids and flavonoid-rich phytochemical preparations that such decreases are attributable to bactericidal activity. Though similar light microscopy studies with 3-O-octanoyl-(-)-epicatechin failed to produce definitively negative results, significant levels of aggregation were not detected in bacterial cells treated with this semi-synthetic flavonoid. Given the above results from time-kill experiments, these findings suggest that the nature of 3-O-octanoyl-(-)-epicatechin activity is bactericidal. In the next phase of the project, attempts were made to ascertain the mechanism(s) of action responsible for the antibacterial activity of galangin. Populations of S. aureus NCTC 6571 incubated for 12 hours in potassium-free media containing 50 mu g/mL galangin were shown to decrease in size by between 0.6 and 1.2 x l0 8 cfu/mL, and lose 19 to 21 % more potassium than untreated control cells. In the same assay, novobiocin caused no increase in potassium loss but penicillin G caused a 4 to 6 % increase in potassium loss. This data may suggest that galangin exerts its antibacterial effect by damaging the cytoplasmic membrane or by damaging the cell wall and causing osmotic lysis. The possibility that galangin has an intracellular mechanism of action that causes potassium loss through autolysis cannot be excluded though. In similar studies with 3-O-octanoyl-(-)-epicatechin, 50 mu g/mL of the acylated flavonoid was found to diminish colony counts of S. aureus NCTC 6571 by 1.6 to 3.0 x l0 9 cfu/mL within 60 minutes of treatment, and cause a 45 to 50 % increase in potassium loss within just 10 minutes. Novobiocin and penicillin G, by comparison, induced no increase in potassium loss during 60 minutes of treatment. This data appears to confirm that 3-O-octanoyl-(-)-epicatechin exerts its primary antibacterial effect by causing direct damage to the cytoplasmic membrane.","abstract_html":"Many bacteria have become resistant to existing drugs and there is now an urgent need for new antibacterial agents. Flavonoids are a class of natural product with a diverse range of pharmacological properties, and previous studies at The Robert Gordon University and University of London have identified several candidates suitable for antibacterial analysis. The purpose of the present research programme was to determine the range of bacteria against which these agents are effective, to identify suitable conditions for in vitro analysis of the most active compounds, to try and characterise flavonoid activity as either bacteriostatic or bactericidal, and to initiate an investigation into the underlying antibacterial mechanism(s) of action of these compounds. In the initial phase of the project, the agents polyphenon E (a green tea extract), galangin, apigenin and baicalin (naturally occurring flavonoids), and 3-O-octanoyl-(-)-epicatechin (a semi-synthetic flavonoid), were screened for inhibitory activity against antibiotic sensitive and resistant strains of Staphylococcus aureus, Enterococcus faecalis, Enterococcus faecium, Escherichia coli and Pseudomonas aeruginosa. Polyphenon E was found to have MIC values of 200 mu g/mL against S. aureus strains, and galangin was found to have MIC values of 50 mu g/mL or less against S. aureus strains, when tested using the broth microdilution and agar dilution methods respectively. MIC values of approximately 50 mu g/mL were determined for 3-O-oclanoyl-(-)- epicatechin against strains of S aureus, and MICs of 100 mu g/mL were determined for this flavonoid against strains of E. faecalis and E. faecium using the broth microdilution method. Little or no activity was detected from the remaining flavonoids when tested at concentrations up to and including 200 mu g/mL. The structural stability of antibacterial flavonoids in solution was then investigated by scanning spectrophotometry. When galangin was dissolved in aqueous sodium carbonate and incubated for 24 hours at 37°C, shifts in wavelength maxima were detected and the flavonoid’s absorbance at these values decreased to as little as 2% of their original value. The alteration in galangin’s structure, implied from these changes, was accompanied by loss of antibacterial activity. Spectrophotometric analysis indicated structural changes when galangin was incubated in aqueous ammonia too. By contrast, galangin’s wavelength maxima did not change during incubation in buffered or unbuffered aqueous DMSO and BSA solution. The wavelength maxima of 3-O-octanoyl-(-)-epicatechin also remained steady when incubated in aqueous DMSO or aqueous ethanol. Decreases in absorbance at wavelength maxima were noted when galangin and 3-O-octanoyl-(-)-epicatechin were incubated in these aqueous organic solutions, but the decreases were small and probably attributable to poor water solubility rather than poor structural stability. When time-kill experiments were performed, 50 mu g/mL galangin was found to cause decreases of 1000-fold and more in colony forming unit numbers of antibiotic sensitive S. aureus and methicillin resistant S. aureus within 60 and 120 minutes incubation respectively. Bacterial re-growth was noted after 3 to 6 hours however. At a concentration of 100 mu g/mL, 3-O-octanoyl-(-)-epicatechin reduced colony counts of both strains by more than 1000-fold in just 60 minutes. Counts were reduced below minimum detectable levels (~50 cfu/mL) within a further 60 minutes, and remained below minimum detectable levels for the duration of the 7 day experiments. In subsequent light microscopy studies, performed using the above results and results from additional time-kill assays as a guide, galangin was shown to cause aggregation of bacterial cells. These results indicate that galangin-induced decreases in colony counts of S. aureus observed in time-kill assays are caused by cell aggregation or a combination of cell aggregation and cell death, and call into question the validity of claims from past investigations with natural flavonoids and flavonoid-rich phytochemical preparations that such decreases are attributable to bactericidal activity. Though similar light microscopy studies with 3-O-octanoyl-(-)-epicatechin failed to produce definitively negative results, significant levels of aggregation were not detected in bacterial cells treated with this semi-synthetic flavonoid. Given the above results from time-kill experiments, these findings suggest that the nature of 3-O-octanoyl-(-)-epicatechin activity is bactericidal. In the next phase of the project, attempts were made to ascertain the mechanism(s) of action responsible for the antibacterial activity of galangin. Populations of S. aureus NCTC 6571 incubated for 12 hours in potassium-free media containing 50 mu g/mL galangin were shown to decrease in size by between 0.6 and 1.2 x l0 8 cfu/mL, and lose 19 to 21 % more potassium than untreated control cells. In the same assay, novobiocin caused no increase in potassium loss but penicillin G caused a 4 to 6 % increase in potassium loss. This data may suggest that galangin exerts its antibacterial effect by damaging the cytoplasmic membrane or by damaging the cell wall and causing osmotic lysis. The possibility that galangin has an intracellular mechanism of action that causes potassium loss through autolysis cannot be excluded though. In similar studies with 3-O-octanoyl-(-)-epicatechin, 50 mu g/mL of the acylated flavonoid was found to diminish colony counts of S. aureus NCTC 6571 by 1.6 to 3.0 x l0 9 cfu/mL within 60 minutes of treatment, and cause a 45 to 50 % increase in potassium loss within just 10 minutes. Novobiocin and penicillin G, by comparison, induced no increase in potassium loss during 60 minutes of treatment. This data appears to confirm that 3-O-octanoyl-(-)-epicatechin exerts its primary antibacterial effect by causing direct damage to the cytoplasmic membrane.","abstract_has_math":false,"creators":["Cushnie, T.P. Tim"],"institution":"Robert Gordon University","degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["A. Lamb and D. Chapman"],"committee_chairs":[],"committee_members":[],"year":2006,"date_issued":"2006","date_published":"2006","updated_at":"2026-07-24T04:10:09Z","subjects":["Antibacterial","Antibiotics","Flavonoids","Bacteriostatic","Bactericidal","Staphylococcus aureus","Wavelength maxima","Cytoplasmic membrane"],"languages":["en"],"rights":[],"rights_urls":[],"identifier_entries":[{"key":"dc:identifier","label":"Identifier","values":["oai:rgu-repository.worktribe.com:2807349","https://doi.org/10.48526/rgu-wt-2807349"],"render_values":[{"text":"oai:rgu-repository.worktribe.com:2807349","href":null,"code":true},{"text":"https://doi.org/10.48526/rgu-wt-2807349","href":"https://doi.org/10.48526/rgu-wt-2807349","code":true}]}]},"links":{"outbound_url":"https://rgu-repository.worktribe.com/2807349/1/CUSHNIE%202006%20Investigation%20of%20the%20antibacterial","outbound_label":"Repository record","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["A. 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Flavonoids are a class of natural product with a diverse range of pharmacological properties, and previous studies at The Robert Gordon University and University of London have identified several candidates suitable for antibacterial analysis. The purpose of the present research programme was to determine the range of bacteria against which these agents are effective, to identify suitable conditions for in vitro analysis of the most active compounds, to try and characterise flavonoid activity as either bacteriostatic or bactericidal, and to initiate an investigation into the underlying antibacterial mechanism(s) of action of these compounds. In the initial phase of the project, the agents polyphenon E (a green tea extract), galangin, apigenin and baicalin (naturally occurring flavonoids), and 3-O-octanoyl-(-)-epicatechin (a semi-synthetic flavonoid), were screened for inhibitory activity against antibiotic sensitive and resistant strains of Staphylococcus aureus, Enterococcus faecalis, Enterococcus faecium, Escherichia coli and Pseudomonas aeruginosa. Polyphenon E was found to have MIC values of 200 mu g/mL against S. aureus strains, and galangin was found to have MIC values of 50 mu g/mL or less against S. aureus strains, when tested using the broth microdilution and agar dilution methods respectively. MIC values of approximately 50 mu g/mL were determined for 3-O-oclanoyl-(-)- epicatechin against strains of S aureus, and MICs of 100 mu g/mL were determined for this flavonoid against strains of E. faecalis and E. faecium using the broth microdilution method. Little or no activity was detected from the remaining flavonoids when tested at concentrations up to and including 200 mu g/mL. The structural stability of antibacterial flavonoids in solution was then investigated by scanning spectrophotometry. When galangin was dissolved in aqueous sodium carbonate and incubated for 24 hours at 37°C, shifts in wavelength maxima were detected and the flavonoid’s absorbance at these values decreased to as little as 2% of their original value. The alteration in galangin’s structure, implied from these changes, was accompanied by loss of antibacterial activity. Spectrophotometric analysis indicated structural changes when galangin was incubated in aqueous ammonia too. By contrast, galangin’s wavelength maxima did not change during incubation in buffered or unbuffered aqueous DMSO and BSA solution. The wavelength maxima of 3-O-octanoyl-(-)-epicatechin also remained steady when incubated in aqueous DMSO or aqueous ethanol. Decreases in absorbance at wavelength maxima were noted when galangin and 3-O-octanoyl-(-)-epicatechin were incubated in these aqueous organic solutions, but the decreases were small and probably attributable to poor water solubility rather than poor structural stability. When time-kill experiments were performed, 50 mu g/mL galangin was found to cause decreases of 1000-fold and more in colony forming unit numbers of antibiotic sensitive S. aureus and methicillin resistant S. aureus within 60 and 120 minutes incubation respectively. Bacterial re-growth was noted after 3 to 6 hours however. At a concentration of 100 mu g/mL, 3-O-octanoyl-(-)-epicatechin reduced colony counts of both strains by more than 1000-fold in just 60 minutes. Counts were reduced below minimum detectable levels (~50 cfu/mL) within a further 60 minutes, and remained below minimum detectable levels for the duration of the 7 day experiments. In subsequent light microscopy studies, performed using the above results and results from additional time-kill assays as a guide, galangin was shown to cause aggregation of bacterial cells. These results indicate that galangin-induced decreases in colony counts of S. aureus observed in time-kill assays are caused by cell aggregation or a combination of cell aggregation and cell death, and call into question the validity of claims from past investigations with natural flavonoids and flavonoid-rich phytochemical preparations that such decreases are attributable to bactericidal activity. Though similar light microscopy studies with 3-O-octanoyl-(-)-epicatechin failed to produce definitively negative results, significant levels of aggregation were not detected in bacterial cells treated with this semi-synthetic flavonoid. Given the above results from time-kill experiments, these findings suggest that the nature of 3-O-octanoyl-(-)-epicatechin activity is bactericidal. In the next phase of the project, attempts were made to ascertain the mechanism(s) of action responsible for the antibacterial activity of galangin. Populations of S. aureus NCTC 6571 incubated for 12 hours in potassium-free media containing 50 mu g/mL galangin were shown to decrease in size by between 0.6 and 1.2 x l0 8 cfu/mL, and lose 19 to 21 % more potassium than untreated control cells. In the same assay, novobiocin caused no increase in potassium loss but penicillin G caused a 4 to 6 % increase in potassium loss. This data may suggest that galangin exerts its antibacterial effect by damaging the cytoplasmic membrane or by damaging the cell wall and causing osmotic lysis. The possibility that galangin has an intracellular mechanism of action that causes potassium loss through autolysis cannot be excluded though. In similar studies with 3-O-octanoyl-(-)-epicatechin, 50 mu g/mL of the acylated flavonoid was found to diminish colony counts of S. aureus NCTC 6571 by 1.6 to 3.0 x l0 9 cfu/mL within 60 minutes of treatment, and cause a 45 to 50 % increase in potassium loss within just 10 minutes. Novobiocin and penicillin G, by comparison, induced no increase in potassium loss during 60 minutes of treatment. This data appears to confirm that 3-O-octanoyl-(-)-epicatechin exerts its primary antibacterial effect by causing direct damage to the cytoplasmic membrane."]},{"key":"dc:title","label":"Title","values":["Investigation of the antibacterial activity of selected flavonoids."]}]}],"canonical_facts":{"dc:contributor.advisor":["A. Lamb and D. Chapman"],"dc:contributor.sponsor":["No Funder Acknowledged (Outputs)"],"dc:creator":["Cushnie, T.P. Tim"],"dc:date":["2006-07-31"],"dc:date.issued":["2006"],"dc:description.abstract":["Many bacteria have become resistant to existing drugs and there is now an urgent need for new antibacterial agents. Flavonoids are a class of natural product with a diverse range of pharmacological properties, and previous studies at The Robert Gordon University and University of London have identified several candidates suitable for antibacterial analysis. The purpose of the present research programme was to determine the range of bacteria against which these agents are effective, to identify suitable conditions for in vitro analysis of the most active compounds, to try and characterise flavonoid activity as either bacteriostatic or bactericidal, and to initiate an investigation into the underlying antibacterial mechanism(s) of action of these compounds. In the initial phase of the project, the agents polyphenon E (a green tea extract), galangin, apigenin and baicalin (naturally occurring flavonoids), and 3-O-octanoyl-(-)-epicatechin (a semi-synthetic flavonoid), were screened for inhibitory activity against antibiotic sensitive and resistant strains of Staphylococcus aureus, Enterococcus faecalis, Enterococcus faecium, Escherichia coli and Pseudomonas aeruginosa. Polyphenon E was found to have MIC values of 200 mu g/mL against S. aureus strains, and galangin was found to have MIC values of 50 mu g/mL or less against S. aureus strains, when tested using the broth microdilution and agar dilution methods respectively. MIC values of approximately 50 mu g/mL were determined for 3-O-oclanoyl-(-)- epicatechin against strains of S aureus, and MICs of 100 mu g/mL were determined for this flavonoid against strains of E. faecalis and E. faecium using the broth microdilution method. Little or no activity was detected from the remaining flavonoids when tested at concentrations up to and including 200 mu g/mL. The structural stability of antibacterial flavonoids in solution was then investigated by scanning spectrophotometry. When galangin was dissolved in aqueous sodium carbonate and incubated for 24 hours at 37°C, shifts in wavelength maxima were detected and the flavonoid’s absorbance at these values decreased to as little as 2% of their original value. The alteration in galangin’s structure, implied from these changes, was accompanied by loss of antibacterial activity. Spectrophotometric analysis indicated structural changes when galangin was incubated in aqueous ammonia too. By contrast, galangin’s wavelength maxima did not change during incubation in buffered or unbuffered aqueous DMSO and BSA solution. The wavelength maxima of 3-O-octanoyl-(-)-epicatechin also remained steady when incubated in aqueous DMSO or aqueous ethanol. Decreases in absorbance at wavelength maxima were noted when galangin and 3-O-octanoyl-(-)-epicatechin were incubated in these aqueous organic solutions, but the decreases were small and probably attributable to poor water solubility rather than poor structural stability. When time-kill experiments were performed, 50 mu g/mL galangin was found to cause decreases of 1000-fold and more in colony forming unit numbers of antibiotic sensitive S. aureus and methicillin resistant S. aureus within 60 and 120 minutes incubation respectively. Bacterial re-growth was noted after 3 to 6 hours however. At a concentration of 100 mu g/mL, 3-O-octanoyl-(-)-epicatechin reduced colony counts of both strains by more than 1000-fold in just 60 minutes. Counts were reduced below minimum detectable levels (~50 cfu/mL) within a further 60 minutes, and remained below minimum detectable levels for the duration of the 7 day experiments. In subsequent light microscopy studies, performed using the above results and results from additional time-kill assays as a guide, galangin was shown to cause aggregation of bacterial cells. These results indicate that galangin-induced decreases in colony counts of S. aureus observed in time-kill assays are caused by cell aggregation or a combination of cell aggregation and cell death, and call into question the validity of claims from past investigations with natural flavonoids and flavonoid-rich phytochemical preparations that such decreases are attributable to bactericidal activity. Though similar light microscopy studies with 3-O-octanoyl-(-)-epicatechin failed to produce definitively negative results, significant levels of aggregation were not detected in bacterial cells treated with this semi-synthetic flavonoid. Given the above results from time-kill experiments, these findings suggest that the nature of 3-O-octanoyl-(-)-epicatechin activity is bactericidal. In the next phase of the project, attempts were made to ascertain the mechanism(s) of action responsible for the antibacterial activity of galangin. Populations of S. aureus NCTC 6571 incubated for 12 hours in potassium-free media containing 50 mu g/mL galangin were shown to decrease in size by between 0.6 and 1.2 x l0 8 cfu/mL, and lose 19 to 21 % more potassium than untreated control cells. In the same assay, novobiocin caused no increase in potassium loss but penicillin G caused a 4 to 6 % increase in potassium loss. This data may suggest that galangin exerts its antibacterial effect by damaging the cytoplasmic membrane or by damaging the cell wall and causing osmotic lysis. The possibility that galangin has an intracellular mechanism of action that causes potassium loss through autolysis cannot be excluded though. In similar studies with 3-O-octanoyl-(-)-epicatechin, 50 mu g/mL of the acylated flavonoid was found to diminish colony counts of S. aureus NCTC 6571 by 1.6 to 3.0 x l0 9 cfu/mL within 60 minutes of treatment, and cause a 45 to 50 % increase in potassium loss within just 10 minutes. Novobiocin and penicillin G, by comparison, induced no increase in potassium loss during 60 minutes of treatment. This data appears to confirm that 3-O-octanoyl-(-)-epicatechin exerts its primary antibacterial effect by causing direct damage to the cytoplasmic membrane."],"dc:identifier":["oai:rgu-repository.worktribe.com:2807349","https://doi.org/10.48526/rgu-wt-2807349"],"dc:identifier.uri":["https://rgu-repository.worktribe.com/2807349/1/CUSHNIE%202006%20Investigation%20of%20the%20antibacterial"],"dc:language":["en"],"dc:publisher.institution":["Robert Gordon University"],"dc:relation.isreferencedby":["https://rgu-repository.worktribe.com/output/2807349"],"dc:subject":["Antibacterial","Antibiotics","Flavonoids","Bacteriostatic","Bactericidal","Staphylococcus aureus","Wavelength maxima","Cytoplasmic membrane"],"dc:title":["Investigation of the antibacterial activity of selected flavonoids."],"dc:type":["Thesis"]},"updated_at":"2026-07-24T04:10:09Z"}