Medicinal Plants Classification Biosynthesis and ... - Index of

Medicinal Plants: A Tool to Overcome Antibiotic Resistance?
As resistances to antimicrobial compounds that disrupt the structure of the bacterial
membranes, rather than inhibiting a specific enzyme, are less likely, such compounds are
considered as the probable future of antibiotics (Lockwood et Mayo, 2003).
3.1.3. Other Mechanisms
Since microorganisms need to adhere to host cells to cause infection, compounds that can
inhibit the adhesion of microbial proteins to host polysaccharide receptors are potential
antimicrobial agents. This mode of action is particularly encountered in lectin molecules and
it is worth emphasizing that such antimicrobial lectins are not detected by using classical
general antimicrobial screening protocols.
The antimicrobial and/or antiparasitic activity of planar molecules, including quaternary
alkaloids (berberine, harmane) is probably due in part to their ability to intercalate within
DNA (Omulokoli et al., 1997).
3.2. Phytochemical Classes of Antimicrobial Compounds
3.2.1. Antibiotic Compounds from Microorganisms
Antibiotics, agents ―against life‖, can either be natural products or synthetic chemicals,
designed to block some crucial process in microbial cells selectively. Most of the antibiotics
introduced into human clinical use to treat infectious disease in the past 60 years have been
natural products, elaborated by one microorganism in a particular habitat and set of
environmental conditions to affect neighboring microbes, either to regulate their growth or to
trigger their elimination (Walsh, 2003). Diverse structures isolated from a series of
microorganisms have yielded the antibiotics, an extremely important class of medicinal
compounds reviewed in details in many papers. Streptomycetes, gram-positive filamentous
bacteria, account for the production of about 55 % of the commercially significant antibiotics
including macrolides, glycopeptides, tetracyclines, β-lactams, aminoglycosides.
Platensimycin, a previously unknown class of antibiotics, is produced by Streptomyces
platensis and shows a strong, broad-spectrum Gram-positive antibacterial activity by
selectively inhibiting cellular lipid biosynthesis (Wang et al., 2006).
3.2.2. Phenolic Compounds
Among the simplest bioactive phytochemicals, simple phenols and phenolic acids
consist of a single, mono- or polysubstituted, phenolic ring and include compounds with
interesting antimicrobial activity. For example, caffeic acid is effective against viruses,
bacteria and fungi, catechol, pyrogallol and arbutine are shown to be toxic to
microorganisms; the site and number of hydroxyl groups are thought to be related to the
relative toxicity to microorganisms, with evidence that increased hydroxylation results in
increased toxicity. The phenolic toxicity to microorganisms can be attributed to their
oxidized, eventually quinone, forms which lead to enzyme inhibition, possibly through
reaction with sulfhydryl and amino groups or through more non-specific interactions with
proteins (Haslam, 1996). The phenylpropanoids (C6-C3), phenolic compounds with a C3 side
chain, are found in essential oils and often cited as antimicrobial agents. Eugenol, a C6-C3
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