Selective Isolation of Rare Actinomycetes Producing Novel Antimicrobial Compounds

Microbial pathogens are developing resistance against existing antibiotics, stressing the urgency for discovery of new therapeutic compounds. Actinomycetes, alone produce 70-80% of the available antibiotics. The chances of isolating undiscovered strains from the terrestrial habitats have diminished so that the search for novel products has switched to rare genera of actinomycetes from normal habitats or to discovery of strains/species found in unusual habitats. Rare genera of actinomycetes can be selectively isolated using various physical and chemical pre-treatment methods. Although preliminary identification of strains can be made on the basis of morphology, yet reliable classification of actinomycetes may not be possible using traditional approaches based on morphological and physiological features alone. Therefore rapid molecular methods including Restriction fragment length polumorphism (RFLP), Pulse field gel electrophoresis (PFGE), Amplified ribosomal DNA restriction analysis (ARDRA), Random amplified polymorphic DNA (RAPD) and genus specific primers would be extremely useful to discern between the rare genera of actinomycetes from the common ones. Primary and secondary screening of isolates is done to determine their antimicrobial potential both qualitatively and quantitatively. Chemical structures of active metabolites are ascertained using chromatographic and spectrophotometric techniques. Thin layer chromatography (TLC) helps in determining the Rf values of the compound fractions while Bioautography helps in pinpointing the exact bioactive fraction(s). Nowadays, TLC and spraying reagent techniques have been replaced by more efficient techniques for frequent detection of antibiotics. Databases are available for retention times and UV-visible absorbance spectra of standards. Therefore, culture filtrates and/or extracts from broth cultures of novel actinomycetes can be compared with those of databases to determine the novelty of compounds and to avoid rediscovery of already known compounds.