3.3 Microbial Metabolites
A number of ‘microbial metabolites’ produced by well-defined process of fermentation give rise to certain very useful therapeutically potent drugs, especially the antibiotics and related antieoplastic agents as exemplified below:
(a) As Antibiotics: for instance:
(i) Chloromycetin – from Streptomyces venezualae Bartz,
(ii) Erythromycin – from Streptomyces erythreus (Walksman) Walksan & Henrici,
(iii) Gentamycin – from Micromonospora purpurea MJ Weinstein et al.
(iv) Penicillin O – from Penicillium chrysogenum,
(v) Streptomycin – from Streptomyces griseus (Krainsky) Walksman et Henrici,
(vi) Tetracycline – from Streptomyces viridifaciens.
(b) As Antineoplastic Agents: for examples:
(i) Dactinomycin – from several Streptomyces spp.
(ii) Daunorubicin – from Strepomyces peucetius G. Cassinelli; P. orezzi.
(iii) Mitomycin C – from Streptomyces caespitosus (griseovinaceseus)
(iv) Pilcamycin (or Mithramycin) – from Streptomyces argillaceus n. sp. and S. tanashiensis
Figure 1.3, illustrates the outline of the fermentation process usually accomplished in a pharmaceutical industry whereby dried drugs are produced in a large scale. However, in certain specific instances, per se cephalosporins, the end product obtained by the fermentation process is routed through semisynthetic means to yield the desired pharmaceutical substance.
It is pertinent to mention here that the production of ‘genetically–engineered–drugs’, bears fundamentally a close resemblance to the various fermentation processes normally employed for the antbiotcs. The major noteworthy difference in this specific instance lies in the fact that a gene
controlling the production of the desired constituent is virtually transferred from its basic source to a fast-growing microbial cell-line whereby permitting the large-scale production in comparatively a much shorter duration.
However, it is rather a ‘difficult task’ to isolate a gene coding for a particular antibiotic. Interestingly, in the actinomycetin fungi, the ‘gene’ was separated conveniently from the chromosomal genes and cloned on naturally occurring plasmids. It has been observed that though plasmids are found in streptomycetes, only in the specific case of methylenomycin* biosynthesis, the extrachromosomal element essentially consists of several structural gene absolutely necessary for the prediction of antibiotic.
In general, a number of methods are employed to identify clones that usually harbour the plasmids carrying antibiotic-biosynthetic genes, namely:
(a) Mutants that are found to be blocked at different steps in the aminoglycosidic-productionpathways are known and also available. These ‘blocked mutants’ may be employed as recipients for the prediction of respective genes from shotgun-cloning-experiments. Shotgun cloning is the isolation of a specific DNA sequence and subsequent screening for the desired phenotype. The plasmids eventually isolated from the transformants, wherein antibioticbiosynthesis is restored by the cloned genes, would ultimately the introduced for maximizing the final yield.
(b) The latest technique of insertional mutagenesis may be used effectively to obtain not only
the mutant but also the cloned DNA in a single experiment.
* Member of a family of cyclopentenoid antibiotic related structurally to sarkomycins, and having in vitro activity Vs
Gram positive and Gram negative organism.
(c) As the enzymes that are intimately involved in the biosynthesis of aminology essentially possess relatively wider substrate specificities, the transfer of genes between such species that cause the production of various aminoglycosides were invariably utilised to generate newer antibiotics. If genes that code for the synthesis of chosen precursors are duly cloned interspecifically many existing aminologycosides may be produced by just a one-step-fermentation process. Mutasynthesis* has paved the way for the introduction of a plethora of interesting hybrids, for instance; mutamicins, hybrimycins and hydroxygentamycin, and
(d) Conversion of Amikacin (I) from Kanamycin (II):
Amikacin (I) is one of the most effective aminoglycosides. It may be produced chemically from Kanamycin (II) but this route is rather expensive and not cost-effective. However, an aminoglycoside producing strain of Bacillus circulars is capable of converting (II) into (I) by the addition of hydroxyaminobutyric acid. Thus, the interspecific transfer of this gene may be used to persuade successfully a kanamycin producing streptomycetes to afford (I) and this recombinant DNA route could prove to be an economical one.
Figure 1.4, summarizes the conversion of Amikacin (I) from Kanamycin (II) via chemical and recombinant DNA routes.
REFERENCES
Ashutosh Kar (2003), Pharmacognosy and Pharmaco biotechnology, 2nd Edition
‘Handbook of Medicinal Herbs’ (2001), J.A. Duke, CRC-Press, London, 1st Edn.
William Charles Evans (2002), Trease and Evans Pharmacognosy 15th Edition by: Trease, Bailliere Tindall; Evans.
Ramstad (1956), E., ‘Modern Pharmacognosy’, McGraw Hill, London.
