Medicinal Plants Classification Biosynthesis and ... - Index of

324
Philippe N. Okusa, Caroline Stevigny and Pierre Duez
compound, present in clove oil, is effective against bacteria and fungi and has been widely
used in dentistry.
Flavonoid and stilbene, structures based on a phenylchromane or a diphenylethene, are
well-known phytoalexins, compounds synthesized by plants in response to microbial and
fungal infections. It should not be surprising that they have been found to be effective against
a wide array of microorganisms. The antimicrobial activity of teas is related to their content
in catechins, reduced form of the C3 unit of flavonoids. These compounds are effective
against many microorganisms including Vibrio cholerae, Streptococcus mutans, Shigella sp.
Flavonoids are also effective against many viruses; swertifrancheside and chrysin have been
found active against HIV; other flavone derivatives show an inhibitory effect on respiratory
syncytial virus (RSV) (Cushnie et Lamb, 2005). Resveratrol, a diphenylethene phytoalexin
commonly found in food and drinks, including red wine, grapes, and peanuts; is effective
against bacteria and fungi.
Tannins, polymeric phenolic substances, characterized by their astringency, are capable
of tanning leather or precipitating gelatin from solution. They are divided into three groups,
(i) the hydrolysable tannins, based on gallic and ellagic acids usually as multiple esters with
polyols; (ii) the procyanidols or condensed tannins which are oligo- and polymerized
flavonoid monomers; and (iii) the complex tannins, comprising monomers from the other 2
classes. Many physiological activities, such as stimulation of phagocytic cells, host-mediated
antitumor activity and antimicrobial effects, have been assigned to tannins. Indeed, in many
studies, tannins showed toxicity to yeasts, filamentous fungi and bacteria; and the
antibacterial effect of some medicinal plants extracts is explained by their content in tannins.
It is also reported that tannins have an inhibitory effect on viral reverse transcriptases
(Scalbert, 1991).
Coumarins, phenolic substances made of fused benzene and α-pyrone rings, are
particularly known for their antithrombotic, anti-inflammatory and vasodilatatory activities.
Warfarin is a well-known coumarin which is used both as an oral anticoagulant and,
interestingly, as a rhodenticide; it may also have antiviral effects. Several other coumarins
have shown antimicrobial activity against many fungi, gram-positive bacteria and viruses. An
indirect negative effect on infections has been observed with some coumarins that stimulate
macrophages.
The potential antimicrobial activity of quinones, aromatic rings with two ketone
substitutions, oxidized phenols ubiquitous in nature and characteristically highly
reactive,toward proteins has been discussed in Section 3.1.1. Anthraquinones, among which
hypericin, isolated from Hypericum perforatum, are effective against many microorganisms
(Cowan, 1999). Arbutin, a hydroquinone derivate, is effective against Pseudomonas
aeruginosa. In Pyrus ussuriensis, arbutin is metabolized to hydroquinone, then to
benzoquinone; this later compound is the substance essential for antibacterial activity of
Pyrus spp.
3.2.2. Terpenoids
Many studies report the antibacterial, antifungal, antiviral and anti protozoal activities of
terpenes and terpenoids, ubiquitous compounds synthesized from isoprene units and occuring
as monoterpenes, sesquiterpenes, diterpenes, triterpenes and tetraterpenes according to their