Medicinal Plants Classification Biosynthesis and ... - Index of

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2.7. General Toxicity Tests
Philippe N. Okusa, Caroline Stevigny and Pierre Duez
To assess the selectivity of the observed antimicrobial activity, cytotoxicity assays on
mammalian cells is very important and should be included in parallel. A MRC-5 cells model
is frequently used; cell proliferation and viability is assessed either by visual counting or by
spectrophotometry after addition of MTT, Alamar BlueTM or resazurin (McMillian et al.,
2002).
3. Plant Compounds with Direct
Antimicrobial Activity
Natural products from plants are a source of ―lead‖ compounds in the search for new
antimicrobial drugs and medicines. Over the past three decades, researchers have also turned
to many of the traditional folk medicines, essentially cocktails of natural products, to uncover
the scientific basis of their remedial effects, endeavors which have their roots in a desire to
improve the efficacy of modern medical practice (Haslam, 1996). In this context, particular
attention has been given to the traditional herbal medicines which provide antimicrobial
compounds with newly discovered mechanisms of action. Table 1 lists examples of some
antimicrobial compounds from medicinal plants.
3.1. Mechanisms of Direct Antimicrobial Action
3.1.1. Association with Macromolecules
The antimicrobial properties of some phytochemicals, notably polyphenols, may be
attributed to their ability to form complexes with macromolecules such as proteins and
polysaccharides. This propensity to bind proteins can presumably lead to an inhibition of
enzymes. Assessment of the medical significance of inhibition of a particular enzyme,
determined in vitro, is however dependant on the absorption and distribution of administered
compounds to the desired in vivo site of action. Such limitations do not arise on local
applications, especially when the enzymes are extracellular. Polyphenols for example are
known to bind glycosyl transferases, enzymes involved in the synthesis of water-insoluble
glucans from sucrose by Streptococcus mutans (Hattori et al., 1990). These glucans firmly
bind the bacteria to the tooth surface, leading eventually to the formation of dental plaque and
development of dental caries; polyphenols extracted from green tea showed inhibitory
activity against glycosyl transferases and their administration led to highly significant
reduction in dental caries in animals (Ooshima et al., 1993).
Antimicrobial quinones also often act by protein binding mechanisms. Indeed, quinones
are known to covalently and irreversibly bind nucleophilic amino acids in proteins, often
leading to inactivation of proteins (Cowan, 1999). Probable targets in the microbial cell are
surface-exposed adhesins, cell wall polypeptides, and membrane-bound enzymes.