Medicinal Plants Classification Biosynthesis and ... - Index of

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Philippe N. Okusa, Caroline Stevigny and Pierre Duez
1. Introduction
Infectious diseases caused by bacteria, fungi, viruses and parasites remain a major threat
to public health due to the emergence of widespread antimicrobial resistance (WHO, 1996)
increasing at an alarming rate (Hawkey, 2000). Thus, it is essential that resistance to
currently used antimicrobial agents be prevented, limited or reversed. Antibiotic
resistance is the ability of microorganisms to remain impervious to the inhibitory or lethal
effect of antibiotics (Kaye et al., 2000). This resistance can be intrinsic, inherent to a
particular species; for example gram-negative bacteria are intrinsically resistant to
vancomycin because these organisms contain an additional protective outer membrane,
absent in gram-positive cells, that prevents the agent from reaching the target site (Walsh,
2003). The resistance can also be acquired, when it refers to an attribute resulting from a
change in the genetic composition of the bacteria, rendering a previously active drug
ineffective (Rice et al., 2003).
If inappropriate prescribing and use of antibiotics are considered as the major factors in
the emergence of the resistance phenomenon, education addressed to both prescribers and
patients can help to reduce resistance (Yates, 1999). A further approach resides in the
research for new drugs, acting by other mechanisms than those described for existing
antibiotics, or inhibiting resistance mechanisms of microorganisms of medical importance.
These new drugs can be provided by natural sources, particularly by medicinal plants.
Indeed, medicinal plants have always provided modern therapeutic a most important source
of lead compounds in the search of new drugs and medicines (Cowan, 1999). Indigenous
herbals are widely used against many infectious diseases and can be a valuable source for
natural compounds, with a great interest in the fight against pathogenic bacteria and
antibiotics resistance. Many studies report the antimicrobial properties of secondary
metabolites from medicinal plants, of which it is estimated that less than 10% of the total
have been characterized (Schultes, 1978). Such compounds are part of "vegetal immunity",
complex mechanisms of plants‘ defenses against predation by microorganisms, insects and
herbivores. Interestingly, some phytochemical compounds, although lacking antimicrobial
activity, display an inhibitory effect on resistance mechanisms of microorganisms, thus
enhancing or restoring the activity of some antibiotics. All these compounds belong to many
phytochemical groups found in several botanical families; .among these, tannins and essential
oils are particularly known for their direct antimicrobial properties.
The antimicrobial activity of plant extracts, essential oils or pure compounds can be
assessed by different methods, including broth dilution (determination of minimum inhibitory
concentrations), diffusion on agar (determination of inhibition diameter) and bioautography
(localization of active compounds on a TLC plate) (Botz et al., 2001). The solvents used for
plant extraction are an important factor in the antimicrobial evaluation; there are solvents
more favorable according to the number of inhibitors extracted (Vanden Berghe et Vlietinck,
1991; Eloff, 1998). The collection and conservation of plant materials is also important to
ensure correct botanical identification and preservation of biological activities.
The present paper reviews (i) the different methods of evaluation of antimicrobial
activity of plant extracts and natural compounds, (ii) the principal groups of direct