278
(Amaral et al. 1998), fungi (Ayafor et al. 1994), viruses (Fujioka et al. 1994) and
protozoa (Ghoshal et al. 1996). Artemisinin, a sesquiterpenoid, and its derivative
α-arteether find their use as antimalarials (Vishwakarma, 2004). Trichorabdal A, a
diterpene from a Japanese herb, is reported to inhibit Helicobacter pylori (Kadota
et al. 1997).
Two pungent compounds in capsicum species (capsaicin and dihydrocapsaicin)
were studied for their antimicrobial activities. The plain and heated extracts showed
inhibition of Bacillus subtilis, B. cereus, Clostridium tetani, C. sporogenes and
Streptococcus pyogenes (Cichewicz and Thorpe 1996). Capsaicin, a terpenoid constituent, is bactericidal to H. pylori (Jones et al. 1997). Another hot-tasting diterpene, aframodial, from a Cameroonian spice, shows broad-spectrum antifungal
activity also (Ayafor et al. 1994)
11.2.9 Essential Oils as Antimicrobials
Essential oils are complex mixtures of volatile secondary metabolites isolated from
plants by steam or hydrodistillation. Essential oils contain a wide variety of secondary metabolites that are capable of inhibiting or slowing the growth of bacteria,
yeasts and moulds. The major constituents of essential oils like monoterpenes, sesquiterpenes and phenylpropanoids are responsible for the fragrance and biological
properties of aromatic medicinal plants (Reichling 1999).
Different concentrations of specific compounds can affect the antimicrobial
potential of essential oils. For example, higher concentrations of eugenol, cinnamaldehyde or citral confer antimicrobial properties to essential oils (Lis-Balchin et al.
1998; Davidson 2001). Eugenol, a phenylpropene, was found to reduce the viability
and resulted in the inactivation of Salmonella typhi. The observed high antibacterial
activity at alkaline pH favours the fact that the compound can work more efficiently
when given in vivo. Eugenol-induced deformation of macromolecules in the membrane and the subsequent antibacterial activity is due to the interaction of eugenol
on bacterial cell membrane (Devi et al. 2010).
The mechanism of action of essential oil depends on the chemical composition
of the bioactive compounds, and their antimicrobial activity is never based on a
unique mechanism but is instead a cascade of reactions encompassing the entire
bacteria (Burt 2004). Essential oil constituents have a variety of targets, especially
the membrane and cytoplasm, and in some cases, they completely alter the cell
morphology (Nazzaro et al. 2013). In general, essential oils act to inhibit bacterial
growth as well as the production of toxic bacterial metabolites. Essential oils possess better activity on Gram-positive bacteria than Gram-negative strains, and this
effect is mostly attributed to the difference in the cell wall compositions
(Chorianopoulos et al. 2008; Gutierrez et al. 2008; Marino et al. 1999).
The cell membrane integrity is essential for the survival of bacteria because it is
a vital factor for the basic biological activities taking place within the cells. In some
cases, essential oils perturb the permeability of membrane by destroying the electron transport system (Tassou et al. 2000). Some of the components of the essential
oils such as carvone, carvacrol and thymol, lead to an increase in the intracellular
R. Reghu et al.
(Amaral et al. 1998), fungi (Ayafor et al. 1994), viruses (Fujioka et al. 1994) and
protozoa (Ghoshal et al. 1996). Artemisinin, a sesquiterpenoid, and its derivative
α-arteether find their use as antimalarials (Vishwakarma, 2004). Trichorabdal A, a
diterpene from a Japanese herb, is reported to inhibit Helicobacter pylori (Kadota
et al. 1997).
Two pungent compounds in capsicum species (capsaicin and dihydrocapsaicin)
were studied for their antimicrobial activities. The plain and heated extracts showed
inhibition of Bacillus subtilis, B. cereus, Clostridium tetani, C. sporogenes and
Streptococcus pyogenes (Cichewicz and Thorpe 1996). Capsaicin, a terpenoid constituent, is bactericidal to H. pylori (Jones et al. 1997). Another hot-tasting diterpene, aframodial, from a Cameroonian spice, shows broad-spectrum antifungal
activity also (Ayafor et al. 1994)
11.2.9 Essential Oils as Antimicrobials
Essential oils are complex mixtures of volatile secondary metabolites isolated from
plants by steam or hydrodistillation. Essential oils contain a wide variety of secondary metabolites that are capable of inhibiting or slowing the growth of bacteria,
yeasts and moulds. The major constituents of essential oils like monoterpenes, sesquiterpenes and phenylpropanoids are responsible for the fragrance and biological
properties of aromatic medicinal plants (Reichling 1999).
Different concentrations of specific compounds can affect the antimicrobial
potential of essential oils. For example, higher concentrations of eugenol, cinnamaldehyde or citral confer antimicrobial properties to essential oils (Lis-Balchin et al.
1998; Davidson 2001). Eugenol, a phenylpropene, was found to reduce the viability
and resulted in the inactivation of Salmonella typhi. The observed high antibacterial
activity at alkaline pH favours the fact that the compound can work more efficiently
when given in vivo. Eugenol-induced deformation of macromolecules in the membrane and the subsequent antibacterial activity is due to the interaction of eugenol
on bacterial cell membrane (Devi et al. 2010).
The mechanism of action of essential oil depends on the chemical composition
of the bioactive compounds, and their antimicrobial activity is never based on a
unique mechanism but is instead a cascade of reactions encompassing the entire
bacteria (Burt 2004). Essential oil constituents have a variety of targets, especially
the membrane and cytoplasm, and in some cases, they completely alter the cell
morphology (Nazzaro et al. 2013). In general, essential oils act to inhibit bacterial
growth as well as the production of toxic bacterial metabolites. Essential oils possess better activity on Gram-positive bacteria than Gram-negative strains, and this
effect is mostly attributed to the difference in the cell wall compositions
(Chorianopoulos et al. 2008; Gutierrez et al. 2008; Marino et al. 1999).
The cell membrane integrity is essential for the survival of bacteria because it is
a vital factor for the basic biological activities taking place within the cells. In some
cases, essential oils perturb the permeability of membrane by destroying the electron transport system (Tassou et al. 2000). Some of the components of the essential
oils such as carvone, carvacrol and thymol, lead to an increase in the intracellular
R. Reghu et al.