key: cord-303879-3hug5hj3 authors: Cushnie, T.P. Tim; Lamb, Andrew J. title: Antimicrobial activity of flavonoids date: 2005-10-19 journal: Int J Antimicrob Agents DOI: 10.1016/j.ijantimicag.2005.09.002 sha: doc_id: 303879 cord_uid: 3hug5hj3 Flavonoids are ubiquitous in photosynthesising cells and are commonly found in fruit, vegetables, nuts, seeds, stems, flowers, tea, wine, propolis and honey. For centuries, preparations containing these compounds as the principal physiologically active constituents have been used to treat human diseases. Increasingly, this class of natural products is becoming the subject of anti-infective research, and many groups have isolated and identified the structures of flavonoids possessing antifungal, antiviral and antibacterial activity. Moreover, several groups have demonstrated synergy between active flavonoids as well as between flavonoids and existing chemotherapeutics. Reports of activity in the field of antibacterial flavonoid research are widely conflicting, probably owing to inter- and intra-assay variation in susceptibility testing. However, several high-quality investigations have examined the relationship between flavonoid structure and antibacterial activity and these are in close agreement. In addition, numerous research groups have sought to elucidate the antibacterial mechanisms of action of selected flavonoids. The activity of quercetin, for example, has been at least partially attributed to inhibition of DNA gyrase. It has also been proposed that sophoraflavone G and (−)-epigallocatechin gallate inhibit cytoplasmic membrane function, and that licochalcones A and C inhibit energy metabolism. Other flavonoids whose mechanisms of action have been investigated include robinetin, myricetin, apigenin, rutin, galangin, 2,4,2′-trihydroxy-5′-methylchalcone and lonchocarpol A. These compounds represent novel leads, and future studies may allow the development of a pharmacologically acceptable antimicrobial agent or class of agents. Resistance to antimicrobial agents has become an increasingly important and pressing global problem. Of the 2 million people who acquire bacterial infections in US hospitals each year, 70% of cases now involve strains that are resistant to at least one drug [1] . A major cause for concern in the UK is methicillin-resistant Staphylococcus aureus (MRSA), which was at low levels a decade ago but now accounts for ca. 50% of all S. aureus isolates [2] . Substantial investment and research in the field of anti-infectives are now desperately needed if a public health crisis is to be averted. Structural modification of antimicrobial drugs to which resistance has developed has proven to be an effective means of extending the lifespan of antifungal agents such as the azoles [3] , antiviral agents such as the non-nucleoside reverse transcriptase inhibitors [4] , and various antibacterial agents including ␤-lactams and quinolones [5] . It is not surprising then, that in response to antimicrobial resistance, major pharmaceutical companies have tended to concentrate their efforts on improving antimicrobial agents in established classes [6] . However, with the portfolio of chemotherapeutics currently available, it has been acknowledged that researchers are getting close to the end game in terms of parent structure alterations. A call has therefore been made for the development of new classes of drug that work on different target sites to those in current use [7, 8] . Rational drug design does not always yield effective antimicrobials. In the past, potent enzyme inhibitors have been successfully designed and synthesised but they had only modest antibacterial activity, probably owing to the complex issue of drug uptake by cells. Broad empirical screening of chemical entities for antimicrobial activity represents an alternative strategy for the development of novel drugs. Natural products have been a particularly rich source of anti-infective agents, yielding, for example, the penicillins in 1940, the tetracyclines in 1948 and the glycopeptides in 1955 [9] . The following review will examine the antimicrobial activity of flavonoids, a class of natural products possessing a diverse range of pharmacological properties. Compounds with antifungal, antiviral and antibacterial activity will each be discussed in turn, with particular emphasis on those flavonoids with antibacterial activity. Flavonoids are ubiquitous in photosynthesising cells and therefore occur widely in the plant kingdom [10] . They are found in fruit, vegetables, nuts, seeds, stems and flowers as well as tea, wine [11] , propolis and honey [12] , and represent a common constituent of the human diet [13] . In the US, the daily dietary intake of mixed flavonoids is estimated to be in the range 500-1000 mg, but this figure can be as high as several grams for people supplementing their diets with flavonoids or flavonoid-containing herbal preparations [14] . The function of flavonoids in flowers is to provide colours attractive to plant pollinators [11, 15] . In leaves, these compounds are increasingly believed to promote physiological survival of the plant, protecting it from, for example, fungal pathogens and UV-B radiation [13, 15] . In addition, flavonoids are involved in photosensitisation, energy transfer, the actions of plant growth hormones and growth regulators, control of respiration and photosynthesis, morphogenesis and sex determination [11, 13] . The basic structural feature of flavonoid compounds is the 2-phenyl-benzo[␣]pyrane or flavane nucleus, which consists of two benzene rings (A and B) linked through a heterocyclic pyrane ring (C) (Fig. 1 ) [16] . Flavonoids can be classified according to biosynthetic origin. Some classes, for example chalcones, flavanones, flavan-3-ols and flavan-3,4-diols, are both intermediates in biosynthesis as well as end products that can accumulate in plant tissues. Other classes are only known as end products of biosynthesis, for example anthocyanidins, proanthocyanidins, flavones and flavonols. Two additional classes of flavonoid are those in which the 2-phenyl side chain of flavanone isomerises to the 3 position, giving rise to isoflavones and related isoflavonoids. The neoflavonoid is formed through further isomerisation to the 4 position [13] . Structures of the major classes of flavonoids are given in Fig. 2 . The structures of specific compounds within these classes that possess antimicrobial activity and that are discussed in the present review are summarised in Table 1 . Individual flavonoids may be assigned names in three different ways. Trivial names are employed extensively and sometimes indicate flavonoid class or plant source. For example, names ending in 'inidin' can denote an anthocyanidin, names ending in 'etin' generally denote a flavonol, and compounds tricin and hypolaetin have been extracted from plants belonging to the genera Triticum and Hypolaena. Flavonoids may also be named in a semi-systematic manner based on trivial names such as flavone or chalcone as the parent structure, e.g. 3,5,7,3 4 -pentahydroxyflavone or 3,3 ,4 ,5,7-pentahydroxyflavone. Lastly, flavonoids may be given systematic chemical names, e.g. 3,4-dihydro-2-phenyl-2H-1-benzopyran for flavan, but this method is cumbersome and rarely used [13] . In the present review, trivial names will be used wherever possible. Increasingly, flavonoids are becoming the subject of medical research. They have been reported to possess many useful properties, including anti-inflammatory activity, oestrogenic activity, enzyme inhibition, antimicrobial activity [10, 13] , antiallergic activity, antioxidant activity [11] , vascular activity and cytotoxic antitumour activity [15] . For a group of compounds of relatively homogeneous structure, the flavonoids inhibit a perplexing number and variety of eukaryotic enzymes and have a tremendously wide range of activities. In the case of enzyme inhibition, this has been postulated to be due to the interaction of enzymes with different parts of the flavonoid molecule, e.g. carbohydrate, phenyl ring, phenol and benzopyrone ring [10] . Several reviews have been written on the interaction between flavonoids and mammalian cells, including comprehensive articles by Harborne and Williams [15] and Middleton et al. [20] . An extensive review on the biochemistry and medical significance of flavonoids has also recently been produced by Havsteen [21] . For centuries, preparations that contain flavonoids as the principal physiologically active constituents have been used by physicians and lay healers in attempts to treat human diseases [10] . For example, the plant Tagetes minuta (containing quercetagetin-7-arabinosyl-galactoside) has been used extensively in Argentine folk medicine to treat infectious disease [22] . The healing properties of propolis (or 'tzori' in Hebrew) are referred to throughout the Old Testament [23] , and this balm was prescribed by Hippocrates (460-377 BC) in Ancient Greece for the treatment of sores and ulcers [24] . [17, 18] , isoflavones [10] , chalcones [13, 19] , flavanones [10, 13] , flavones [10] , flavonols [10] , flavanon-3-ols [13] , anthocyanidins [13, 20] , flavan-3-ols [10, 13] , proanthocyanidins (occur as dimers, trimers, tetramers and pentamers; R = 0, 1, 2 or 3 flavan-3-ol structures) [13] , flavans [13] , flavan-3,4-diols [13] and dihydrochalcones [13] . Table 1 A summary of the structures of antimicrobial flavonoids discussed within the present review article (compiled from The Handbook of Natural Flavonoids [13] and individual research papers) Substituents at carbon position: 2 3 4 5 6 7 8 2 3 4 5 6 Flavones and their glycosides Acacetin Galangin Flavan-3,4-diols and anthocyanidins Leucocyanidin - The antimicrobial properties of propolis have been attributed to its high flavonoid content and in particular the presence of the flavonoids galangin and pinocembrin [12, [25] [26] [27] . Huangchin (Scutellaria baicalensis) is yet another example. This herbal medicine has been used systemically and topically for thousands of years in China for the treatment of periodontal abscesses and infected oral wounds. The flavone baicalein is reported to be largely responsible for this plant's antimicrobial effects [28] . A - - - OH R3 OH R3 - - OH - - Naringenin - - - OH - OH - - - OH - - Naringin - - - OH - OR7 - - - OH - - Pinocembrin - - - OH - OH - - - - - - Ponciretin - - - OH - OH - - - OCH 3 - - Sophoraflavanone G - - - OH - OH R10 OH - OH - - 3-Methyleneflavanone - CH 2 - - - - - - - - - - 5,7,4 -Trihydroxy-8-methyl-6-(3-methyl-[2- butenyl])-(2S)-flavanone - - - O H R 3 O H C H 3 - - O H - - Chalcones Licochalcone A - R11 OH - OCH 3 - - - - O H - - Licochalcone C - - OH R3 OCH 3 - - - - O H - - 2,4,2 -Trihydroxychalcone OH - OH - - - - O H - - - - 2,4,2 -Trihydroxy-5 -methylchalcone OH - OH CH 3 - - - O H - - - -OH OH OH - OH - - OH OH - - Pelargonidin chloride - Cl - OH - OH - - - OH - - Flavans 6,4 -Dichloroflavan - - - - C l - - - - C l - - 7-Hydroxy-3 ,4 -(methylenedioxy)flavan - - - - - O H - - # # - - R1 It has been suggested that because flavonoids are widely distributed in edible plants and beverages and have previously been used in traditional medicine, they are likely to have minimal toxicity. However, this family of compounds has a diverse range of activities in mammalian cells [14, 20] and in vivo confirmation of their side effects would be necessary for a full evaluation of their practical usefulness in the field of modern medicine [29] . Given that the selectivity of flavonoids for eukaryotic enzymes appears to vary from compound to compound [15, 20] , such a study would need to assess the toxicity of these phytochemicals on an individual basis. Owing to the widespread ability of flavonoids to inhibit spore germination of plant pathogens, they have been proposed for use against fungal pathogens of man [15] . A new prenylated flavanone recently isolated from the shrub Eysenhardtia texana has been identified as 5,7,4 -trihydroxy-8methyl-6-(3-methyl-[2-butenyl])-(2S)-flavanone and shown to possess activity against the opportunistic pathogen Candida albicans [30] . The flavonoid 7-hydroxy-3 ,4 -(methylenedioxy)flavan, isolated from Terminalia bellerica fruit rind, has also been shown to possess activity against C. albicans [31] . Two new flavones from Artemisia giraldi, identified as 6,7,4 -trihydroxy-3 ,5 -dimethoxyflavone and 5,5dihydroxy-8,2 ,4 -trimethoxyflavone, together with 5,7,4 -trihydroxy-3 ,5 -dimethoxyflavone have been reported to exhibit activity against Aspergillus flavus [32] , a species of fungi that causes invasive disease in immunosuppressed patients [33] . The activity of propolis against dermatophytes and Candida spp. has been attributed at least partially to its high flavonoid content [34] . Galangin, a flavonol commonly found in propolis samples [24] , has been shown to have inhibitory activity against Aspergillus tamarii, A. flavus, Cladosporium sphaerospermum, Penicillium digitatum and Penicillium italicum [35] . A recent area of research that is of particular interest is the apparent inhibitory activity of some flavonoids against human immunodeficiency virus (HIV). To date, most if not all investigations have involved work with the pandemic HIV-1 strain and its enzymes. In vitro studies have shown that baicalin inhibits HIV-1 infection and replication. Inhibition of HIV-1 entry into cells expressing CD4 and chemokine co-receptors [36] , and antagonism of HIV-1 reverse transcriptase [37] by the flavone O-glycoside have been demonstrated by Li and colleagues. Baicalein [38] , robustaflavone and hinokiflavone [39] have also been shown to inhibit HIV-1 reverse transcriptase, as have several catechins, but catechins inhibit other DNA polymerases and their interaction with the HIV-1 enzyme is therefore thought to be non-specific in nature [40] . In addition, it has been demonstrated that several flavonoids, including demethylated gardenin A and 3,2 -dihydroxyflavone, inhibit HIV-1 proteinase [41] . Robinetin, myricetin, baicalein, quercetagetin [42] and quercetin 3-O-(2 -galloyl)-␣-l-arabinopyranoside [43] inhibit HIV-1 integrase, although there are concerns that HIV enzyme inhibition by quercetagetin and myricetin is non-specific [44] . It has also been reported that the flavonoids chrysin, acacetin and apigenin prevent HIV-1 activation via a novel mechanism that probably involves inhibition of viral transcription [45] . Interestingly, in a study by Hu and colleagues, chrysin was reported to have the highest therapeutic index of 21 natural and 13 synthetic flavonoids against HIV-1 [46] . Several research groups have investigated the relationship between flavonoid structure and inhibitory activity against HIV-1 and its enzymes [39, 41, 42, 44, 46] . Furthermore, at least two groups have proposed mechanisms of action for HIV-1 enzyme inhibition [41, 42] . Flavonoids also have inhibitory activity against a variety of other viruses. For example, Selway reports that quercetin, morin, rutin, dihydroquercetin, dihydrofisetin, leucocyanidin, pelargonidin chloride and catechin possess activity against up to seven types of virus, including herpes simplex virus (HSV), respiratory syncytial virus, poliovirus and Sindbis virus [11, 47] . Proposed antiviral mechanisms of action include inhibition of viral polymerase and binding of viral nucleic acid or viral capsid proteins [47] . In addition to the flavonoids mentioned above, three proanthocyanidins from Pavetta owariensis (with structural similarity to proanthocyanidin A2 and cinnamtannin B1 and B2) have been shown to have activity against HSV and coxsackie B virus [48, 49] . It has also been demonstrated that two of the flavonoids found in propolis, chrysin and kaempferol, inhibit viral replication of HSV, human coronavirus and rotavirus [50] . More recently, the flavonol galangin has been reported to have significant antiviral activity against HSV and coxsackie B virus [51] . Although naturally occurring flavonoids with antiviral activity have been recognised since the 1940s, it is only in the last 25 years that attempts have been made to synthetically modify flavonoids for improved antiviral activity. One such synthesised compound is 6,4 -dichloroflavan. However, despite showing strong in vitro activity, this compound proved unsuccessful in clinical trials [11] . Synergism has been demonstrated between various combinations of flavones and flavonols. For example, kaempferol and luteolin show synergy against HSV. It has been suggested that this is why propolis is more active than its individual component compounds [52] . Synergism has also been reported between flavonoids and other antiviral agents. Quercetin, for example, potentiates the effects of 5-ethyl-2 -dioxyuridine [11] and acyclovir [53] against HSV and pseudorabies infection. Apigenin also enhances the antiviral activity of acyclovir against these viruses [53] . The antibacterial activity of flavonoids is being increasingly documented. Crude extracts from plants with a history of use in folk medicine have been screened in vitro for antibacterial activity by many research groups. Flavonoidrich plant extracts from species of Hypericum [54] , Capsella [55] and Chromolaena [55] have been reported to possess antibacterial activity. Many other phytochemical preparations with high flavonoid content have also been reported to exhibit antibacterial activity [22, [56] [57] [58] [59] [60] [61] [62] [63] . Propolis has been analysed on many occasions too, and samples containing high concentrations of flavonoids are frequently reported to show antibacterial activity [12, [25] [26] [27] 50, 64] . Many research groups have gone one step further and either isolated and identified the structure of flavonoids that possess antibacterial activity, or quantified the activity of commercially available flavonoids. Examples of such flavonoids are apigenin [65] [66] [67] [68] [69] [70] [71] [72] [73] , galangin [35, [74] [75] [76] [77] , pinocembrin [78, 79] , ponciretin [80, 81] , genkwanin [66, 82] , sophoraflavanone G and its derivatives [29, [83] [84] [85] , naringin and naringenin [29, 60, 86, 87] , epigallocatechin gallate and its derivatives [74, [88] [89] [90] [91] [92] [93] [94] [95] , luteolin and luteolin 7glucoside [69, 73, 96] , quercetin, 3-O-methylquercetin and various quercetin glycosides [60, 65, 72, 87, [97] [98] [99] [100] [101] [102] and kaempferol and its derivatives [60, 65, 74, 76, 87, 98, 100, 103] . Other flavones [32, 60, 74, [104] [105] [106] [107] , flavone glycosides [86, 108, 109] , isoflavones [110, 111] , flavanones [29, 30, 78, 79, 104, [111] [112] [113] [114] , isoflavanones [115] , isoflavans [116] , flavonols [74, 114, 117] , flavonol glycosides [86, [118] [119] [120] and chalcones [79, 104, 111, 121] with antibacterial activity have also been identified. Some researchers have reported synergy between naturally occurring flavonoids and other antibacterial agents against resistant strains of bacteria. Examples of these include epicatechin gallate [122] [123] [124] [125] and sophoraflavanone G [83, 84] . At least one group has demonstrated synergy between flavonoids with antibacterial activity [126] . Others have synthetically modified natural flavones and analysed them for antibacterial activity [94, [127] [128] [129] [130] [131] . For example, Wang and colleagues have complexed 5-hydroxy-7,4 -dimethoxyflavone with a number of transition metals and shown that this process increases antibacterial activity [130] . Another group reported increased antibacterial activity of 3-methyleneflavanones when the B ring contained bromine or chlorine substituents [131] . Two research groups have described the use of flavonoids in vivo. In a study by Vijaya and Ananthan, oral administration of either 142.9 mg/kg quercetin or 214.3 mg/kg quercetrin protected guinea pigs against an induced Shigella infection that killed untreated control animals [132] . More recently, Dastidar and co-workers reported that intraperitoneal injection of either 1.58 mg/kg sophoraisoflavone A or 3.16 mg/kg 6,8diprenylgenistein gave significant protection to mice challenged with ∼9.5 × 10 8 colony-forming units (CFUs) of Salmonella typhimurium [110] . When reports of the antibacterial activity of flavonoids are compared, the results appear widely conflicting (Table 2) . For example, it was published that apigenin had no activity against S. aureus at concentrations up to 128 g/mL [72] ; a separate study in the same year reported that the flavone inhibited the growth of 15 strains of MRSA and 5 sensitive strains of S. aureus at concentrations between 3.9 g/mL Table 2 The inhibitory activity of apigenin against numerous species of Gram-positive and + + DD, disk diffusion assay; BMAD, broth macrodilution assay; BMID, broth microdilution assay; NS, assay type not stated in report; AWD, agar well diffusion assay; AD, agar dilution assay; +, antibacterial activity detected; −, no antibacterial activity detected. and 15.6 g/mL [73] . From Table 2 it can be seen that such discrepancies could perhaps be attributed on occasion to different assays being used (e.g. [65, 70] and [72, 73] ). Many different assays are employed in flavonoid research, including the agar dilution technique [29] , the paper disk diffusion assay [115] , the hole-plate diffusion method [22] , the cylinder diffusion method [60] , the broth macrodilution technique [71] and the broth microdilution technique [134] . In particular, assays relying on diffusion of test flavonoids may not give a reliable quantitative measure of antibacterial activity because a potent antibacterial flavonoid may have a low rate of diffusion [32] . However, it is clear from Table 2 that additional factors are involved in causing these discrepancies because even groups using the same assay are obtaining conflicting results (e.g. [67, 96] and [67, 72] ). Such inconsistencies may be due to variations within each assay. For example, different groups using the agar dilution technique have used different sizes of bacterial inoculum [81, 86] . In a report by the National Committee for Clinical Laboratory Standards (NCCLS), inoculum size was considered the single most important variable in susceptibility testing [135] . It should be noted that many groups assaying flavonoid antibacterial activity have not quantified the test bacterial suspension [60, 115] and others have not even standardised the size of their unenumerated inocula [35, 56, 76, 90, 97] . From the published work it is clear that, in addition to inoculum size, there are many other variable factors for each type of assay. These include volume of broth or agar [90, 116] , type of broth or agar [86, 92] , size of wells [56, 60] , size of paper disks [57, 65] , strains of a particular bacterial species used [69, 72] and incubation period [90, 116] . Recently, a set of guidelines was published for standard agar dilution, broth macrodilution and broth microdilution methods [136] . This may help to reduce the number of conflicting reports of flavonoid antibacterial activity in the future. However, it will remain necessary to consider carefully additional variables such as the solvent used to dissolve test flavonoids [116, 118] . It has previously been shown that precipitation occurs when selected flavonoids are dissolved in organic solvents and diluted with neutral polar solutions [75] . Precipitation of flavonoids in a minimum inhibitory concentration (MIC) assay is likely to cause diminished contact between bacterial cells and flavonoid molecules and may lead to false negative reports of antibacterial activity. Also, in improperly controlled experiments, flavonoid precipitation could be misinterpreted as bacterial growth and further false negative results may be recorded as a consequence. The structural alteration of flavonoids such as galangin in alkaline solvents is another matter for consideration [75] . If flavonoid salts are formed and these have increased or decreased potency compared with the parent structure, this may lead to false positive/negative reports of antibacterial activity. Other variables worth noting include whether the test flavonoids are obtained from a commercial or natural source [35, 74] and which companies [74, 75] /natural products [71, 72] the compounds are from. The diverse range of cell functions affected by flavonoids in eukaryotic systems is well documented [10, 20] . Although there have been comparatively few studies into the mechanisms underlying flavonoid antibacterial activity, information from published literature indicates that different compounds within this class of phytochemicals may target different components and functions of the bacterial cell [137] [138] [139] . If this is the case, it is surprising that the small number of groups which have investigated the relationship between flavonoid structure and antibacterial activity (summarised below) have been able to identify common structural features among active compounds. However, it may be that individual antibacterial flavonoids have multiple cellular targets, rather than one specific site of action. Alternatively, these common structural features may simply be necessary for flavonoids to gain proximity to or uptake into the bacterial cell. Tsuchiya and colleagues sought to establish a structureactivity relationship for flavanones by isolating a number of differently substituted compounds and determining their MICs against MRSA [29] . Their study indicated that 2 ,4 -or 2 ,6 -dihydroxylation of the B ring and 5,7-dihydroxylation of the A ring in the flavanone structure was important for anti-MRSA activity. Substitution at the 6 or 8 position with a long chain aliphatic group such as lavandulyl (5-methyl-2isopropenyl-hex-4-enyl) or geranyl (trans-3,7-dimethyl-2,6octadienyl) also enhanced activity [29] . Interestingly, a recent report by Stapleton and colleagues demonstrated that substitution with C 8 and C 10 chains also enhanced the antistaphylococcal activity of flavonoids belonging to the flavan-3-ol class [94] . Osawa et al. assessed the activity of a number of structurally different flavonoids including flavones, flavanones, isoflavones and isoflavanones based on the paper disk agar diffusion assay [115] . It was shown that 5-hydroxyflavanones and 5-hydroxyisoflavanones with one, two or three additional hydroxyl groups at the 7, 2 and 4 positions inhibited the growth of Streptococcus mutans and Streptococcus sobrinus. These results correlate well with those of Tsuchiya and colleagues [29] . It was also reported by Osawa and colleagues that 5-hydroxyflavones and 5-hydroxyisoflavones with additional hydroxyl groups at the 7 and 4 positions did not exhibit this inhibitory activity [115] . However, when Sato et al. examined two isoflavones with hydroxyl groups at the 5, 2 and 4 positions using an agar dilution assay, intensive inhibitory activity was detected against a wide range of streptococcal species [107] . This may suggest that hydroxylation at position 2 is important for activity. Alternatively, the lack of activity detected by Osawa et al. may simply be due to the poor diffusion of flavones and isoflavones (compared with flavanones and isoflavanones) through the medium. A more recent paper [104] also reports the importance of a hydroxyl group at position 5 of flavanones and flavones for activity against MRSA, supporting the earlier findings of Tsuchiya et al. [29] . It further states that chalcones are more effective against MRSA than flavanones or flavones, and that hydroxyl groups at the 2 position are important for the antistaphylococcal activity of these compounds. Methoxy groups were reported to drastically decrease the antibacterial activity of flavonoids [104] . The importance of hydroxylation at the 2 position for antibacterial activity of chalcones is supported by earlier work from Sato and colleagues, who found that 2,4,2 -trihydroxy-5 -methylchalcone and 2,4,2trihydroxychalcone inhibited the growth of 15 strains of cariogenic streptococci [140] . As mentioned previously, Ward and colleagues synthesised a number of halogenated derivatives of 3methyleneflavanone [131] . Substitution of the B ring was found to enhance antibacterial activity, with 3 -chloro, 4chloro and 4 -bromo analogues each being approximately twice as effective as their parent compound against S. aureus, and four times more active against Enterococcus faecalis. Also, the 2 ,4 -dichloro derivative exhibited a four-to eightfold improvement in activity against S. aureus and a twoto four-fold improvement against E. faecalis. By contrast, 3-methylene-6-bromoflavanone was less potent than the parent compound and the authors suggested that halogenation of the A ring may diminish activity [131] . Clearly, however, it would be necessary to prepare analogues with substitution at other A-ring positions before this could be said with any certainty. In chalcones, neither fluorination nor chlorination at position 4 of the B ring is reported to affect antibacterial potency significantly [104] . Again, however, other structural analogues of this class of flavonoids would need to be synthesised and examined before the effect of halogenation upon antibacterial activity could be properly assessed. Several research groups have attempted to determine whether flavonoid activity is bacteriostatic or bactericidal by conducting time-kill studies. In such experiments, epigallocatechin gallate [89] , galangin [75] and 3-O-octanoyl-(+)-catechin [94] have been shown to cause a reduction of 1000-fold or more in viable counts of MRSA-YK, S. aureus NCTC 6571 and EMRSA-16, respectively. This would immediately appear to suggest that flavonoids are capable of bactericidal activity. However, it has recently been demonstrated that 3-O-octanoyl-(−)-epicatechin induces the formation of pseudomulticellular aggregates both in antibiotic-sensitive and antibiotic-resistant strains of S. aureus [94] . If this phenomenon is induced by other compounds within the flavonoid class (and liposomal studies suggest that this is the case for epigallocatechin gallate [88] ), questions are raised regarding the interpretation of results from time-kill studies. It may be that flavonoids are not killing bacterial cells but merely inducing the formation of bacterial aggregates and thereby reducing the number of CFUs in viable counts. In a study using radioactive precursors, Mori and colleagues showed that DNA synthesis was strongly inhibited by flavonoids in Proteus vulgaris, whilst RNA synthesis was most affected in S. aureus [138] . Flavonoids exhibiting this activity were robinetin, myricetin and (−)-epigallocatechin. Protein and lipid synthesis were also affected but to a lesser extent. The authors suggested that the B ring of the flavonoids may play a role in intercalation or hydrogen bonding with the stacking of nucleic acid bases and that this may explain the inhibitory action on DNA and RNA synthesis [138] . Ohemeng et al. screened 14 flavonoids of varying structure for inhibitory activity against Escherichia coli DNA gyrase, and for antibacterial activity against Staphylococcus epidermidis, S. aureus, E. coli, S. typhimurium and Stenotrophomonas maltophilia [68] . It was found that E. coli DNA gyrase was inhibited to different extents by seven of the compounds, including quercetin, apigenin and 3,6,7,3 ,4pentahydroxyflavone. Interestingly, with the exception of 7,8-dihydroxyflavone, enzyme inhibition was limited to those compounds with B-ring hydroxylation [68, 141] . The authors proposed that the observed antibacterial activity of the seven flavonoids was due in part to their inhibition of DNA gyrase. However, since the level of antibacterial activity and enzyme inhibition did not always correlate, they also suggested that other mechanisms were involved [68] . More recently, Plaper and colleagues reported that quercetin binds to the GyrB subunit of E. coli DNA gyrase and inhibits the enzyme's ATPase activity [142] . Enzyme binding was demonstrated by isolating E. coli DNA gyrase and measuring quercetin fluorescence in the presence and absence of the gyrase subunits. The flavonoid-binding site was postulated to overlap with those of ATP and novobiocin, since addition of these compounds interfered with quercetin fluorescence. Inhibition of GyrB ATPase activity by quercetin was also demonstrated in a coupled ATPase assay. This research is in agreement with the earlier findings of Ohemeng et al. [68] and supports the suggestion that quercetin's antibacterial activity against E. coli may be at least partially attributable to inhibition of DNA gyrase. When screening natural products for type II topoisomerase inhibitors, Bernard and co-workers found that the glycosylated flavonol rutin was very effective [143] . This compound exhibited antibacterial activity against a permeable E. coli strain (a strain into which the envA1 allele had been incorporated [144, 145] ). Using enzyme assays and a technique known as the SOS chromotest, it was shown that rutin selectively promoted E. coli topoisomerase IV-dependent DNA cleavage, inhibited topoisomerase IV-dependent decatenation activity and induced the SOS response of the E. coli strain. The group suggested that since topoisomerase IV is essential for cell survival, the rutin-induced topoisomerase IV-mediated DNA cleavage leads to an SOS response and growth inhibition of E. coli cells [143] . Within our own laboratory, a 4-quinolone-resistant S. aureus strain was shown to have increased susceptibility to the flavonol galangin compared with other 4-quinolonesensitive and -resistant strains [146] . Interestingly, this strain possesses a distinct amino acid substitution (serine to proline) at position 410 of the GrlB subunit. This may suggest that topoisomerase IV and the relatively homologous gyrase enzyme are involved in the antibacterial mechanism of action of galangin. Clearly, however, further work with mutant strains and purified enzymes would be necessary before this could be verified. A research team that had previously found sophoraflavanone G to have intensive antibacterial activity against MRSA and streptococci [83] [84] [85] recently reported attempts to elucidate the mechanism of action of this flavanone [139] . The effect of sophoraflavanone G on membrane fluidity was studied using liposomal model membranes and compared with the less active flavanone naringenin, which lacks 8-lavandulyl and 2 -hydroxyl groups. At concentrations corresponding to the MIC values, sophoraflavanone G was shown to increase fluorescence polarisation of the liposomes significantly. These increases indicated an alteration of membrane fluidity in hydrophilic and hydrophobic regions, suggesting that sophoraflavanone G reduced the fluidity of outer and inner layers of membranes. Naringenin also exhibited a membrane effect but at much higher concentrations. This correlation between antibacterial activity and membrane interference was suggested to support the theory that sophoraflavanone G demonstrates antibacterial activity by reducing membrane fluidity of bacterial cells [139] . Another group, Ikigai and colleagues, carried out research on (−)-epigallocatechin gallate, a strongly antibacterial catechin found in green tea. Catechins are a group of flavonoids that appear to have greater activity against Gram-positive than Gram-negative bacteria [88] . In this study, liposomes were again used as model bacterial membranes, and it was shown that epigallocatechin gallate induced leakage of small molecules from the intraliposomal space. Aggregation was also noted in liposomes treated with the compound. The group therefore concluded that catechins primarily act on and damage bacterial membranes. It was not known how this damage occurred but two theories were put forward. First, catechins may perturb the lipid bilayers by directly penetrating them and disrupting the barrier function. Alternatively, catechins may cause membrane fusion, a process that results in leakage of intramembranous materials and aggregation. Interestingly, the group also demonstrated that leakage induced by epigallocatechin gallate was significantly lower when liposome membranes were prepared containing negatively charged lipids. It was therefore suggested that the low catechin susceptibility of Gram-negative bacteria may be at least partially attributable to the presence of lipopolysaccharide acting as a barrier [88] . As mentioned previously, Stapleton and colleagues found that substitution with C 8 and C 10 chains increased the antibacterial activity of selected flavan-3-ols (catechins). The group went on to show that cells of an MRSA clinical isolate treated with (−)-epicatechin gallate and 3-O-octanoyl-(+)-catechin, respectively, exhibited moderately and highly increased levels of labelling with the selectively permeable fluorescent stain propidium iodide. In addition, when S. aureus cells were grown in the presence of either (−)-epicatechin gallate or 3-O-octanoyl-(−)-epicatechin and examined by transmission electron microscopy, they were shown to form pseudomulticellular aggregates [94] . This work constitutes a substantial advance in the development of catechins as antibacterial agents and lends support to Ikigai's argument that catechins act on and damage bacterial membranes. It has also been demonstrated by Sato and colleagues that the chalcone 2,4,2 -trihydroxy-5 -methylchalcone induces leakage of 260 nm absorbing substances from S. mutans. This observation generally indicates leakage of intracellular material such as nucleotide, and the authors suggested that 2,4,2 -trihydroxy-5 -methylchalcone exerts its antibacterial effect by changing the permeability of the cellular membrane and damaging membrane function [140] . In addition, the effect of galangin upon cytoplasmic integrity in S. aureus has been investigated by measuring loss of internal potassium [147] . When high cell densities of S. aureus were incubated for 12 h in media containing 50 g/mL of the flavonol, a 60-fold decrease in the number of CFUs was noted and cells lost ca. 20% more potassium than untreated control bacteria. These data strongly suggest that galangin induces cytoplasmic membrane damage and potassium leakage. Whether galangin damages the membrane directly, or indirectly as a result of autolysis or cell wall damage and osmotic lysis, remains to be established however [147] . In an investigation into the antimicrobial action of propolis, Mirzoeva and colleagues showed that one of its constituent flavonoids, quercetin, caused an increase in permeability of the inner bacterial membrane and a dissipation of the membrane potential [148] . The electrochemical gradient of protons across the membrane is essential for bacteria to maintain capacity for ATP synthesis, membrane transport and motility. Mirzoeva et al. suggested that the effect of propolis on membrane permeability and membrane potential may contribute enormously to its overall antibacterial activity and may decrease the resistance of cells to other antibacterial agents. It was thought that this might explain the synergistic effect that occurs between propolis and other antibiotics such as tetracycline [148] and ampicillin [149] . The group also demonstrated that the flavonoids quercetin and naringenin significantly inhibited bacterial motility, providing further evidence that the proton motive force is disrupted. Bacterial motility and chemotaxis are thought to be important in virulence as they guide bacteria to their sites of adherence and invasion. Mirzoeva et al. suggested that the antimotility action of propolis components may have an important role in inhibition of bacterial pathogenesis and the development of infection [148] . The cytoplasmic membrane activity detected for quercetin by Mirzoeva and co-workers may represent one of the additional mechanisms of antibacterial action that was suspected to be present among the seven DNA gyrase-inhibiting flavonoid compounds tested by Ohemeng and colleagues [68] . Haraguchi and colleagues recently carried out an investigation into the antibacterial mode of action of two retrochalcones (licochalcone A and C) from the roots of Glycyrrhiza inflata [137] . These flavonoids demonstrated inhibitory activity against S. aureus and Micrococcus luteus but not against E. coli, and in preliminary tests licochalcone A inhibited incorporation of radioactive precursors into macromolecules (DNA, RNA and protein). The group hypothesised that the licochalcones may be interfering with energy metabolism in a similar way to respiratory-inhibiting antibiotics, since energy is required for active uptake of various metabolites and for biosynthesis of macromolecules [137] . Interestingly, the licochalcones were found to inhibit strongly oxygen consumption in M. luteus and S. aureus but not in E. coli, which correlated well with the observed spectrum of antibacterial activity. The group further demonstrated that licochalcones A and C effectively inhibited NADH-cytochrome c reductase, but not cytochrome c oxidase or NADH-CoQ reductase. It was therefore suggested that the inhibition site of these retrochalcones was between CoQ and cytochrome c in the bacterial respiratory electron transport chain [137] . Merck Research Laboratories recently reported that the flavanone lonchocarpol A inhibits macromolecular synthesis in Bacillus megaterium. Using radioactive precursors, it was demonstrated that RNA, DNA, cell wall and protein synthesis were all inhibited at concentrations similar to the MIC value [150] . This may represent another example of a flavonoid that interferes with energy metabolism. With regard to natural products, it is generally accepted that phytochemicals are less potent anti-infectives than agents of microbial origin, i.e. antibiotics [48] . However, new classes of antimicrobial drug are urgently required and the flavonoids represent a novel set of leads. Future optimisation of these compounds through structural alteration may allow the development of a pharmacologically acceptable antimicrobial agent or group of agents. Existing structure-activity data suggest that it might be possible, for example, to prepare a potent antibacterial flavanone by synthesising a compound with halogenation of the B ring as well as lavandulyl or geranyl substitution of the A ring. Also, it is worth noting that the rapid progress which is being made toward elucidation of flavonoid biosynthetic pathways [151] may soon allow the production of structural analogues of active flavonoids through genetic manipulation. Screening of these analogues might lead to the identification of compounds that are sufficiently potent to be useful as antifungal, antiviral or antibacterial chemotherapeutics. In addition to the structural alteration of weak and moderately active antimicrobial flavonoids, investigation into the mechanisms of action of these compounds is likely to be a productive area of research. Such information may assist in the optimisation of a lead compound's activity, provide a focus for toxicological attention and aid in the anticipation of resistance. Also, characterisation of the interaction between antimicrobial flavonoids and their target sites could potentially allow the design of secondgeneration inhibitors. Infectious Diseases Society of America. Statement of the IDSA concerning 'Bioshield II: Responding to an ever-changing threat Pharmageddon: is it too late to tackle growing resistance to anti-infectives? New developments in anti-HIV chemotherapy Overcoming antimicrobial resistance by targeting resistance mechanisms New ways to treat bacterial infections The global threat of antibiotic resistance (British Pharmaceutical Conference 2000) Antiviral resistance: mechanisms, clinical significance, and future implications Screening of natural products for antimicrobial agents Flavonoids, a class of natural products of high pharmacological potency The impact of plant flavonoids on mammalian biology: implications for immunity, inflammation and cancer Antibacterial properties of propolis (bee glue) The handbook of natural flavonoids Potential health impacts of excessive flavonoid intake Advances in flavonoid research since 1992 A review of the genetic effects of naturally occurring flavonoids, anthraquinones and related compounds Comparison of binding interactions of dibromoflavonoids with transthyretin Application of microwave in organic synthesis; dry synthesis of 2-arylmethylene-3(2)-naphthofuranones Activity of plant flavonoids against antibioticresistant bacteria The effects of plant flavonoids on mammalian cells: implications for inflammation, heart disease, and cancer The biochemistry and medical significance of the flavonoids Antimicrobial activity of flavonoids from leaves of Tagetes minuta Jeremiah 8, verse 22 Bee propolis In vitro activity of propolis against Streptococcus pyogenes Abd Allah FA. Chemical composition and antimicrobial activity of European propolis Growth inhibition of Bacillus subtilis and composition of various propolis extracts Effect of Chinese and western antimicrobial agents on selected oral bacteria Comparative study on the antibacterial activity of phytochemical flavanones against methicillin-resistant Staphylococcus aureus Antibacterial and antifungal flavanones from Eysenhardtia texana New anti-HIV-1, antimalarial, and antifungal compounds from Terminalia bellerica Two flavones from Artemisia giraldii and their antimicrobial activity UK: WCB/McGraw-Hill Antifungal activity of Apulia region propolis The antimicrobial activity of 3,5,7-trihydroxyflavone isolated from the shoots of Helichrysum aureonitens Flavonoid baicalin inhibits HIV-1 infection at the level of viral entry Inhibition of HIV infection by baicalin -a flavonoid compound purified from Chinese herbal medicine Inhibition of reverse transcriptase activity by a flavonoid compound, 5,6,7-trihydroxyflavone In vitro anti-HIV activity of biflavonoids isolated from Rhus succedanea and Garcinia multiflora Observations on the inhibition of HIV-1 reverse transcriptase by catechins Flavones are inhibitors of HIV-1 proteinase Inhibition of HIV-1 integrase by flavones, caffeic acid phenethyl ester (CAPE) and related compounds A new flavonol glycoside gallate ester from Acer okamotoanum and its inhibitory activity against human immunodeficiency virus-1 (HIV-1) integrase Differential inhibitory effects of various flavonoids on the activities of reverse transcriptase and cellular DNA and RNA polymerases Inhibition of HIV activation in latently infected cells by flavonoid compounds Acacetin-7-O-beta-D-galactopyranoside, an anti-HIV principle from Chrysanthemum morifolium and a structure-activity correlation with some related flavonoids Antiviral activity of flavones and flavans Natural products of commercial potential as medicines Plant antiviral agents. VII. Antiviral and antibacterial proanthocyanidins from the bark of Pavetta owariensis Honey bee propolis: prospects in medicine Antiviral activity of galangin isolated from the aerial parts of Helichrysum aureonitens Synergistic effect of flavones and flavonols against herpes simplex virus type 1 in cell culture. Comparison with the antiviral activity of propolis Combined effects of flavonoids and acyclovir against herpesviruses in cell cultures Antimicrobial activity of some Hypericum species Preliminary screening of some Egyptian weeds for antimicrobial activity Preliminary biological and phytochemical investigation of two Nigerian medicinal plants Phytochemical and biological studies of medicinal plants in Bahrain: family Chenopodiaceae. Part 2 In vitro antimicrobial activity of Salsola rosmarinus and Adiantum capillus-veneris Antimicrobial activity of flowers from Anthemis cotula Antimicrobial effects of Finnish plant extracts containing flavonoids and other phenolic compounds Antimicrobial activity of Elaeocarpus sphaericus Antimicrobial activity of the plant Cirsium oleraceum (L.) Scop Flavonoids from Gnaphalium gracile H.B.K Preparation of water and ethanolic extracts of propolis and evaluation of the preparations Antimicrobial principles from tissue culture of some plant species Genus Baccharis. II. Antimicrobial activity of B. crispa and B. notosergila 6-Methoxylated and C-glycosyl flavonoids from Centaurea species DNA gyrase inhibitory and antibacterial activity of some flavones (1) Flavonoids of Limonium axillare Flavones and sesquiterpene lactones from Achillea atrata subsp. multifida: antimicrobial activity Antibacterial activity of pure flavonoids isolated from mosses Antibacterial and allelopathic activity of extract from Castanea sativa leaves Phytochemical flavones isolated from Scutellaria barbata and antibacterial activity against methicillin-resistant Staphylococcus aureus Antibacterial activity of flavonoids against Staphylococcus epidermidis, a skin bacterium Assessment of the antibacterial activity of selected flavonoids and consideration of discrepancies between previous reports Antimicrobial constituents of Gomphrena martiana and Gomphrena boliviana Galangin expresses bactericidal activity against multiple-resistant bacteria: MRSA, Enterococcus spp. and Pseudomonas aeruginosa Two antimicrobial flavanones from the leaves of Glycyrrhiza glabra Antimicrobial activities of constituents of Uvaria chamae In vitro anti-Helicobacter pylori activity of some flavonoids and their metabolites Anti-Helicobacter pylori activity of the metabolites of poncirin from Poncirus trifoliata by human intestinal bacteria Antimicrobial evaluation of coumarins and flavonoids from the stems of Daphne gnidium L Anti-MRSA activity of sophoraflavanone G and synergism with other antibacterial agents Antibacterial activity of flavanone isolated from Sophora exigua against methicillin-resistant Staphylococcus aureus and its combination with antibiotics Inhibition of the growth of cariogenic bacteria in vitro by plant flavanones Examination of coumarins, flavonoids and polysaccharopeptide for antibacterial activity Antibacterial action of some naturally occurring citrus bioflavonoids Bactericidal catechins damage the lipid bilayer Antibacterial activity of epigallocatechin gallate against methicillin-resistant Staphylococcus aureus Antibacterial substances in Japanese green tea extract against Streptococcus mutans, a cariogenic bacterium Microbiological activity of whole and fractionated crude extracts of tea (Camellia sinensis), and of tea components Anti-Helicobacter pylori activity of Chinese tea: in vitro study Mechanism of synergy between epigallocatechin gallate and beta-lactams against methicillin-resistant Staphylococcus aureus Anti-Staphylococcus aureus activity and oxacillin resistance modulating capacity of 3-O-acyl-catechins Antimicrobial activity of 10 different plant polyphenols against bacteria causing food-borne disease Antibacterial activity studies of flavonoids from Salvia palaestina Isolation of antimicrobial compounds from guava (Psidium guajava L.) and their structural elucidation Antimicrobial activities of some flavonoid compounds Antibacterial activity of Achyrocline flaccida Antimicrobial principles from in vitro tissue culture of Tribulus alatus Isolation of flavonoids and a chalcone from Helichrysum odoratissimum and synthesis of helichrysetin Quercetin 3-O-galactosyl-(1-6)-glucoside, a compound from narrowleaf vetch with antibacterial activity Isolation and antimicrobial activity of flavonoids from Prunus spinosa L. flowers Antibacterial activity of flavonoids against methicillin-resistant Staphylococcus aureus strains Two new flavones from Calliandra californica Antibacterial flavonoids from Helichrysum picardii and H. italicum Flavones with antibacterial activity against cariogenic bacteria A new flavone glycoside with antimicrobial activity from Carduus pycnocephalus L A rare antibacterial flavone glucoside from Lantana camara Studies on the antibacterial potentiality of isoflavones Antimicrobial and radical scavenging flavonoids from the stem wood of Erythrina latissima Antimicrobial activity of two flavanones isolated from the Cameroonian plant Erythrina sigmoidea New antimicrobial flavanones from Physena madagascariensis Antibacterial and antiandrogen flavonoids from Sophora flavescens Isoflavanones from the heartwood of Swartzia polyphylla and their antibacterial activity against cariogenic bacteria Antimicrobial flavonoids from Glycyrrhiza glabra hairy root cultures Structure and biological activity of a new rotenoid from Pongamia pinnata Shamimin: a new flavonol C-glycoside from leaves of Bombax ceiba Acylated flavonol glycosides from leaves of Stenochlaena palustris A new bio-active flavonol glycoside from the stems of Butea superba Roxb Antifungal and antibacterial chalcones from Myrica serrata Activity of the tea component epicatechin gallate and analogues against methicillin-resistant Staphylococcus aureus Marked reduction in the minimum inhibitory concentration (MIC) of beta-lactams in methicillin-resistant Staphylococcus aureus produced by epicatechin gallate, an ingredient of green tea (Camellia sinensis) The effect of a component of tea (Camellia sinensis) on methicillin resistance, PBP2 synthesis, and beta-lactamase production in Staphylococcus aureus Modulation of beta-lactam resistance in Staphylococcus aureus by catechins and gallates Rutin-enhanced antibacterial activities of flavonoids against Bacillus cereus and Salmonella enteritidis Synthesis and antimicrobial activity of flavone-6-carboxaldehyde oxime ether derivatives Synthesis and antimicrobial activity of flavone-3 -carboxaldehyde oxime ether derivatives Synthesis and antimicrobial activity of some new flavonyl oxime ether derivatives Synthesis, characterization, and antibacterial activity of transition metal complexes with 5-hydroxy-7,4 -dimethoxyflavone Antimicrobial 3-methyleneflavanones Therapeutic efficacy of medicinal plants against experimentally induced shigellosis in guinea pigs Isolation of the antimicrobial alkaloid stemmadenine from Iraqi Rhazya stricta Baicalin synergy with beta-lactam antibiotics against methicillin-resistant Staphylococcus aureus and other beta-lactam-resistant strains of S. aureus Methods for determining bactericidal activity of antimicrobial agents Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically Mode of antibacterial action of retrochalcones from Glycyrrhiza inflata Antibacterial activity and mode of action of plant flavonoids against Proteus vulgaris and Staphylococcus aureus Reduction of membrane fluidity by antibacterial sophoraflavanone G isolated from Sophora exigua Growth inhibition of oral bacteria related to denture stomatitis by anticandidal chalcones A comparison of active site binding of 4-quinolones and novel flavone gyrase inhibitors to DNA gyrase Characterization of quercetin binding site on DNA gyrase Glycosylated flavones as selective inhibitors of topoisomerase IV Transduction and dominance studies of the envA gene present in a chain-forming mutant of Escherichia coli K12 Mutant of Escherichia coli with anomalous cell division and ability to decrease episomally and chromosomally mediated resistance to ampicillin and several other antibiotics Assessment of the antibacterial activity of galangin against 4-quinolone resistant strains of Staphylococcus aureus Detection of galangin-induced cytoplasmic membrane damage in Staphylococcus aureus by measuring potassium loss Antimicrobial action of propolis and some of its components: the effects on growth, membrane potential and motility of bacteria Svabic-Vlahovic M. In vitro antimicrobial activity of propolis and synergism between propolis and antimicrobial drugs Antibacterial activity of lonchocarpol A Prospects for the metabolic engineering of bioactive flavonoids and related phenylpropanoid compounds The authors are very grateful to Dr Paul Kong and Dr Satyajit Sarker for critiquing preliminary drafts of the manuscript and for advice on flavonoid classification and structure. Thanks are extended to Dr Peter Taylor for insightful comments regarding interpretation of data from time-kill studies with flavonoids. Thanks also to Dr Derek Chapman, Miss Vivienne Hamilton, Dr Bruce Thomson and Mrs Amina Al-Mossawi for their kind support and encouragement.