Medicinal Plants Classification Biosynthesis and ... - Index of

Medicinal Plants: A Tool to Overcome Antibiotic Resistance?
antimicrobial compounds from medicinal plants and their mechanisms of action; and (iii) the
natural compounds inhibiting resistance mechanisms of microorganisms.
2. Extraction and Biological Tests of
Medicinal Plants
2.1. Extraction
The screening of medicinal plants for antimicrobial activity follows a logical pathway.
Plants are collected randomly or by following instructions given by traditional healers in the
areas where the plants are found (Verpoorte et al., 2005). Any part of the plant may contain
antimicrobial compounds, for instance, the roots of Albertisia villosa contain antimicrobial
alkaloids (Lohombo-Ekomba et al., 2004), while eucalyptus leaves are harvested for their
essential oils and tannins. It is also possible to use herbarium specimens to test the
antimicrobial activity of medicinal plants (Eloff, 1999).
Crude aqueous or alcohol extracts are typically used for the preliminary antimicrobial
tests; they can be followed by various organic extractions. For alcoholic extracts, plant parts
are dried, ground to a fine texture, and then soaked in methanol or ethanol for extended
periods. The slurry is filtered and the filtrate dried under reduced pressure. Crude extracts can
then be used in agar disk diffusion or broth dilution tests for antimicrobial properties and for
other biological activities. Active crude extracts can be submitted to a bio-guided
fractionation in order to purify and identify the active compounds by various techniques
including chromatography and spectroscopy (Pieters et Vlietinck, 2005).
Eloff has examined the ability to extract antimicrobial compounds from plants of a
variety of solvents, as well as other factors such as their relative rankings as biohazards and
the ease of removal from fractions (Eloff, 1998). Acetone, which is not one of the most
frequently used extractants in published studies, received the highest overall rating, followed
by dichloromethane, methanol, ethanol and water, respectively. This study suggests that most
of the active antimicrobial compounds are not water-soluble and that the most commonly
used solvents may not present the highest sensitivity in initial screenings for antimicrobial
compounds.
2.2. Agar Diffusion Methods
The agar diffusion assay is one of the most commonly used methods for antimicrobials
susceptibility testing. Test compounds at known concentrations are brought in contact with
an inoculated agar medium, the inoculated system is maintained at room temperature for
several hours in order to favor the diffusion of test compounds over the microbial growth
surface. The system is then incubated at requested t° and the diameter of the clear zone
around the disk of test compound ("inhibition diameter") is measured after the incubation
period. The diffusion method is not appropriate for testing non-polar samples or samples that
do not easily diffuse into agar. In general, the relative antimicrobial potency of different
samples may not always be compared, mainly because of differences in physical properties,
such as solubility, volatility and diffusion characteristics in agar (Cos et al., 2006).
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