277
11.2.6 Coumarins
Coumarins are fragrant organic chemical compounds in the benzopyrone chemical
class, which are colourless crystalline substances in its standard state. A natural
substance found in many plants, coumarins are also phenolic compounds consisting
of fused benzene and α-pyrone ring. At low concentrations (between 16 and 125 μg/
mL), coumarins showed significant antibacterial activity against both Gramnegative and Gram-positive bacteria (Adriana Basile et al. 2009). The coumarins,
agasyllin and aegelinol from the roots of Ferulago campestris (Apiaceae), showed
significant antibacterial activity. Warfarin, a coumarin, possesses antiviral effects
(Berkada 1978; Liedtke and Rathbun 2009). Coumarin was also found to inhibit
Candida albicans (Thornes 1997). It has been also used to avoid cold sore recurrences due to HSV-1 in humans. Phytoalexins, which are hydroxylated derivatives
of coumarins, are produced in carrots in response to fungal infection and seem to
have antifungal activity (Hoult and Payá 1996). Hydroxycinnamic acids, related to
coumarins, are inhibitory to Gram-positive bacteria (Fernández et al. 1996).
11.2.7 Polypeptides and Lectins
Plant antimicrobial peptides (AMPs) are a component of the barrier defence system
of plants. They have been isolated from roots, seeds, flowers, stems and leaves of a
wide variety of species and have activities towards phytopathogens, as well as
against bacteria pathogenic to humans. Peptides are often positively charged and
contain disulphide bonds (Zhang and Lewis 1997). Their mode of action may be the
formation of ion channels in the microbial membrane, which results in a state of
disrupted membrane transport (Terras et al. 1993; Zhang and Lewis 1997). Barley
and wheat comprise of peptides called thionins, which are toxic to bacteria as well
as yeasts (Fernandez de Caleya et al. 1972). The antibacterial and antifungal activities of plants like Amaranthus have already been identified (De Bolle et al. 1996).
Recent research has been focused on studying anti-HIV peptides and lectins.
11.2.8 Terpenes and Terpenoids
The terpene compounds are also referred to as isoprenoids and have the general
chemical structure C 10 H 16 . They occur as diterpenes (C 20 ), triterpenes (C 30 ), tetraterpenes (C 40 ), sesquiterpenes (C 15 ) and hemiterpenes (C 5 ). Terpenoids are derivatives
of terpenes containing additional elements, usually oxygen.
Antimicrobial activity of terpenes has been well studied. Totarol, a diterpenoid
phenol, has been reported to inhibit the proliferation of numerous pathogenic Grampositive bacteria like Mycobacterium tuberculosis. Totarol inhibits bacterial proliferation by targeting FtsZ and suppresses the GTPase activity of MtbFtsZ. It may be
beneficial as a lead compound to develop an effective antitubercular drug (Jaiswal
et al. 2007). Terpenes or terpenoids possess a wide range of activity against bacteria
11 Antimicrobial Agents from Plants
11.2.6 Coumarins
Coumarins are fragrant organic chemical compounds in the benzopyrone chemical
class, which are colourless crystalline substances in its standard state. A natural
substance found in many plants, coumarins are also phenolic compounds consisting
of fused benzene and α-pyrone ring. At low concentrations (between 16 and 125 μg/
mL), coumarins showed significant antibacterial activity against both Gramnegative and Gram-positive bacteria (Adriana Basile et al. 2009). The coumarins,
agasyllin and aegelinol from the roots of Ferulago campestris (Apiaceae), showed
significant antibacterial activity. Warfarin, a coumarin, possesses antiviral effects
(Berkada 1978; Liedtke and Rathbun 2009). Coumarin was also found to inhibit
Candida albicans (Thornes 1997). It has been also used to avoid cold sore recurrences due to HSV-1 in humans. Phytoalexins, which are hydroxylated derivatives
of coumarins, are produced in carrots in response to fungal infection and seem to
have antifungal activity (Hoult and Payá 1996). Hydroxycinnamic acids, related to
coumarins, are inhibitory to Gram-positive bacteria (Fernández et al. 1996).
11.2.7 Polypeptides and Lectins
Plant antimicrobial peptides (AMPs) are a component of the barrier defence system
of plants. They have been isolated from roots, seeds, flowers, stems and leaves of a
wide variety of species and have activities towards phytopathogens, as well as
against bacteria pathogenic to humans. Peptides are often positively charged and
contain disulphide bonds (Zhang and Lewis 1997). Their mode of action may be the
formation of ion channels in the microbial membrane, which results in a state of
disrupted membrane transport (Terras et al. 1993; Zhang and Lewis 1997). Barley
and wheat comprise of peptides called thionins, which are toxic to bacteria as well
as yeasts (Fernandez de Caleya et al. 1972). The antibacterial and antifungal activities of plants like Amaranthus have already been identified (De Bolle et al. 1996).
Recent research has been focused on studying anti-HIV peptides and lectins.
11.2.8 Terpenes and Terpenoids
The terpene compounds are also referred to as isoprenoids and have the general
chemical structure C 10 H 16 . They occur as diterpenes (C 20 ), triterpenes (C 30 ), tetraterpenes (C 40 ), sesquiterpenes (C 15 ) and hemiterpenes (C 5 ). Terpenoids are derivatives
of terpenes containing additional elements, usually oxygen.
Antimicrobial activity of terpenes has been well studied. Totarol, a diterpenoid
phenol, has been reported to inhibit the proliferation of numerous pathogenic Grampositive bacteria like Mycobacterium tuberculosis. Totarol inhibits bacterial proliferation by targeting FtsZ and suppresses the GTPase activity of MtbFtsZ. It may be
beneficial as a lead compound to develop an effective antitubercular drug (Jaiswal
et al. 2007). Terpenes or terpenoids possess a wide range of activity against bacteria
11 Antimicrobial Agents from Plants