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11.2.4 Quinones/Naphthoquinones
Quinones are aromatic rings with two ketone substitutions. Quinones are coloured
and contain the same basic chromophore, that of benzoquinone, which is composed
of two carbonyl groups in conjugation with two carbon–carbon double bonds. For
their identification, quinones are divided into four groups: anthraquinones, benzoquinones, naphthaquinones and isoprenoid quinones. They are highly reactive and
are ubiquitous in nature. Quinones are known to form complexes with nucleophilic
amino acids in proteins (Stern et al. 1996) resulting in loss of function. Cell wall
polypeptides, surface-exposed adhesins and membrane-bound enzymes are the possible targets of quinones in the microbial cell. It may also render substrates unavailable to microorganisms (Cowan 1999). Hypericin, an anthraquinone from
Hypericum perforatum, a popular antidepressant, also possesses antimicrobial
property (Duke 1985; Kazmi et al. 1994).
11.2.5 Tannins
Tannins are polyphenols found in almost all plant parts showing antibacterial activity due to the inactivation of bacterial adhesins, transport proteins and vital enzymes
(Savoia 2012). The different possible modes of antimicrobial activity of tannins
include protein binding, enzyme inhibition and substrate deprivation (Scalbert
1991; Haslam 1996).
The antimicrobial activity of tannins can be attributed to the following properties: (1) enzyme inhibition and substrate deprivation, (2) metal ion deprivation and
(3) action on the membranes of the microorganisms (Chung et al. 1998a). This is
evident from the fact that many microbial enzymes in raw culture filtrates as well as
in purified forms are inhibited when mixed with tannins. Gallotannin-rich plant
extracts showed bacterial inhibition which is attributable to the strong affinity for
iron and to the inactivation of membrane-bound proteins (De Pasquale et al. 1995).
Tannic acid, which is present in many foods like tea, grapes and strawberry, was
found to be inhibitory to the growth of intestinal bacteria such as Escherichia coli,
Clostridium perfringens, Enterobacter cloacae and Bacteroides fragilis. Chung
et al. (1998b) found that the inhibitory effect of tannic acid on the growth of intestinal bacteria may be due to its strong iron-binding capacity. However, many
microbes have evolved to resist greater concentration of tannins. Fungi like
Penicillium and Aspergillus, which produce tannases, are good examples of tanninresistant microorganisms (Scalbert 1991).
R. Reghu et al.
11.2.4 Quinones/Naphthoquinones
Quinones are aromatic rings with two ketone substitutions. Quinones are coloured
and contain the same basic chromophore, that of benzoquinone, which is composed
of two carbonyl groups in conjugation with two carbon–carbon double bonds. For
their identification, quinones are divided into four groups: anthraquinones, benzoquinones, naphthaquinones and isoprenoid quinones. They are highly reactive and
are ubiquitous in nature. Quinones are known to form complexes with nucleophilic
amino acids in proteins (Stern et al. 1996) resulting in loss of function. Cell wall
polypeptides, surface-exposed adhesins and membrane-bound enzymes are the possible targets of quinones in the microbial cell. It may also render substrates unavailable to microorganisms (Cowan 1999). Hypericin, an anthraquinone from
Hypericum perforatum, a popular antidepressant, also possesses antimicrobial
property (Duke 1985; Kazmi et al. 1994).
11.2.5 Tannins
Tannins are polyphenols found in almost all plant parts showing antibacterial activity due to the inactivation of bacterial adhesins, transport proteins and vital enzymes
(Savoia 2012). The different possible modes of antimicrobial activity of tannins
include protein binding, enzyme inhibition and substrate deprivation (Scalbert
1991; Haslam 1996).
The antimicrobial activity of tannins can be attributed to the following properties: (1) enzyme inhibition and substrate deprivation, (2) metal ion deprivation and
(3) action on the membranes of the microorganisms (Chung et al. 1998a). This is
evident from the fact that many microbial enzymes in raw culture filtrates as well as
in purified forms are inhibited when mixed with tannins. Gallotannin-rich plant
extracts showed bacterial inhibition which is attributable to the strong affinity for
iron and to the inactivation of membrane-bound proteins (De Pasquale et al. 1995).
Tannic acid, which is present in many foods like tea, grapes and strawberry, was
found to be inhibitory to the growth of intestinal bacteria such as Escherichia coli,
Clostridium perfringens, Enterobacter cloacae and Bacteroides fragilis. Chung
et al. (1998b) found that the inhibitory effect of tannic acid on the growth of intestinal bacteria may be due to its strong iron-binding capacity. However, many
microbes have evolved to resist greater concentration of tannins. Fungi like
Penicillium and Aspergillus, which produce tannases, are good examples of tanninresistant microorganisms (Scalbert 1991).
R. Reghu et al.