Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 1: INTRODUCTIONThe classical whole-cell well agar diffusion assay has been the conventional approachused in the screening of secondary metabolites which has usually been conducted in arandom fashion. The major limitations of this assay is that these test methods areused repeatedly using similar target organisms and common SM classes are often rediscoveredand are restricted to the search only for antiinfectives (Grabley et al.,1999). Although incorporating new target organisms has lead to the discovery of newcompounds (Higashide, 1995), in actinobacteria the rediscovery rate is 99 % (Zähner& Fiedler, 1995). Once a microorganism shows the producing capacity for secondarymetabolites, time consuming taxonomic studies are often required in identifying andcharacterising the microorganism which are costly and labour intensive (Ōmura,1986). Figure 4 depicts a flow chart of the screening process from the potentialproducer species to the recognition of a new bioactive metabolite.Figure 4. Screening process for a new bioactive microbial metabolite (Bérdy, 1989).Rational selection of microorganisms by chemical or genetic fingerprinting isproviding a way to exclude previously isolated organisms from screening programsand proving to overcome the problems of dereplication (Colquhoun et al., 2000;Brandão et al., 2002). Alternative approaches to classical or modifications to SMscreening have identified a number of ways that could lead to the discovery of newcompounds. These approaches include (i) re-evaluation and further development ofsecondary metabolites that have already been commercially introduced; (ii) evaluationof known antibiotics not used for human therapy; (iii) searching new secondarymetabolites using new test methods, novel microorganisms and varying cultureconditions; (iv) focus on isolates from unusual or little-explored ecosystems;_____________________________________________________________________8