275
11.2.3 Flavonoids
Flavonoids are aromatic compounds with phenolic structures which are widely distributed in the plant kingdom. Structurally they are derived from the parent substance flavone. They are classified into several classes, including flavonols, flavones,
isoflavones, glycoflavones, anthocyanins. Some flavonoids occur as glycosides
which are hydrolysed in the human gut to aglycones. Flavonoids also exist as monomers, dimers or oligomers (Cook and Sammam 1996; Cushnie and Lamb 2005) and
are the most abundant polyphenols in our diets (Jin Dai and Russell 2010). They are
abundantly found in many natural sources like fruits, vegetables, nuts, seeds, tea,
flowers, honey and propolis and therefore form part of the normal diet of humans
(Cook and Sammam 1996). Screening of various isoflavonoids isolated from
Erythrina variegata (Leguminosae) for antibacterial activity against methicillinresistant Staphylococcus aureus (MRSA) indicated that erycristagallin
(11a- dehydropterocarpan) and orientanol B (9-hydroxy-3-methoxy-2-γ,γdimethylallylpterocarpan) showed significant anti-MRSA activity (3·13–6·25 μg
ml
−1
) (Tanaka et al. 2002).
Many reports claim the effectiveness of flavonoids in medical conditions, including antimicrobial, anti-inflammatory, oestrogenic, antioxidant and chelating, vascular and antitumour activities (Cook and Sammam 1996; Cushnie and Lamb 2005).
More lipophilic flavones and their derivatives can cause disruption of microbial
membranes (Tsuchiya et al. 1996). The catechins present in green tea exhibited
antibacterial activity against Vibrio cholerae (Borris 1996), Streptococcus mutans
(Sakanaka et al. 1989) and many other microbes (Tsuchiya et al. 1996; Vijaya et al.
1995). Their activity may be attributed to their ability to complex with extracellular
as well as soluble proteins (Cowan 1999). The flavonoids (6-hydroxy7methoxyluteolin) and the xanthones (8-carboxymethyl 1,5,6 trihydroxy-3-methoxy
xanthone) extracted from the leaves of Leiothrix spiralis, a plant belonging to the
Eriocaulaceae family, revealed a considerable activity against Pseudomonas aeruginosa and E. coli (Araujo et al. 2011). Recent reports even indicated significant
antibacterial activity of some flavonoids against Mycobacterium tuberculosis
(García et al. 2012).
Various studies have examined the relationship between the structure of flavonoids and their antibacterial activity, and these are in close agreement with what
have been already reported. The antibacterial mechanism of action of selected flavonoids has been elucidated. For example, the activity of quercetin has been partially attributed to DNA gyrase inhibition. It has also been proposed that
(−)-epigallocatechin gallate and sophoraflavone G inhibit cytoplasmic membrane
function and the licochalcones A and C inhibit energy metabolism. The mechanism
of action of the flavonoids robinetin, myricetin, apigenin, rutin, galangin and lonchocarpol A has also been investigated and identified, and these compounds represent novel drug leads. Future studies may lead to the development of
pharmacologically acceptable antimicrobial agents from these lead compounds
(Cushnie and Lamb 2005).
11 Antimicrobial Agents from Plants
11.2.3 Flavonoids
Flavonoids are aromatic compounds with phenolic structures which are widely distributed in the plant kingdom. Structurally they are derived from the parent substance flavone. They are classified into several classes, including flavonols, flavones,
isoflavones, glycoflavones, anthocyanins. Some flavonoids occur as glycosides
which are hydrolysed in the human gut to aglycones. Flavonoids also exist as monomers, dimers or oligomers (Cook and Sammam 1996; Cushnie and Lamb 2005) and
are the most abundant polyphenols in our diets (Jin Dai and Russell 2010). They are
abundantly found in many natural sources like fruits, vegetables, nuts, seeds, tea,
flowers, honey and propolis and therefore form part of the normal diet of humans
(Cook and Sammam 1996). Screening of various isoflavonoids isolated from
Erythrina variegata (Leguminosae) for antibacterial activity against methicillinresistant Staphylococcus aureus (MRSA) indicated that erycristagallin
(11a- dehydropterocarpan) and orientanol B (9-hydroxy-3-methoxy-2-γ,γdimethylallylpterocarpan) showed significant anti-MRSA activity (3·13–6·25 μg
ml
−1
) (Tanaka et al. 2002).
Many reports claim the effectiveness of flavonoids in medical conditions, including antimicrobial, anti-inflammatory, oestrogenic, antioxidant and chelating, vascular and antitumour activities (Cook and Sammam 1996; Cushnie and Lamb 2005).
More lipophilic flavones and their derivatives can cause disruption of microbial
membranes (Tsuchiya et al. 1996). The catechins present in green tea exhibited
antibacterial activity against Vibrio cholerae (Borris 1996), Streptococcus mutans
(Sakanaka et al. 1989) and many other microbes (Tsuchiya et al. 1996; Vijaya et al.
1995). Their activity may be attributed to their ability to complex with extracellular
as well as soluble proteins (Cowan 1999). The flavonoids (6-hydroxy7methoxyluteolin) and the xanthones (8-carboxymethyl 1,5,6 trihydroxy-3-methoxy
xanthone) extracted from the leaves of Leiothrix spiralis, a plant belonging to the
Eriocaulaceae family, revealed a considerable activity against Pseudomonas aeruginosa and E. coli (Araujo et al. 2011). Recent reports even indicated significant
antibacterial activity of some flavonoids against Mycobacterium tuberculosis
(García et al. 2012).
Various studies have examined the relationship between the structure of flavonoids and their antibacterial activity, and these are in close agreement with what
have been already reported. The antibacterial mechanism of action of selected flavonoids has been elucidated. For example, the activity of quercetin has been partially attributed to DNA gyrase inhibition. It has also been proposed that
(−)-epigallocatechin gallate and sophoraflavone G inhibit cytoplasmic membrane
function and the licochalcones A and C inhibit energy metabolism. The mechanism
of action of the flavonoids robinetin, myricetin, apigenin, rutin, galangin and lonchocarpol A has also been investigated and identified, and these compounds represent novel drug leads. Future studies may lead to the development of
pharmacologically acceptable antimicrobial agents from these lead compounds
(Cushnie and Lamb 2005).
11 Antimicrobial Agents from Plants