Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 4: DISCUSSIONsources used in the nine complex media evaluated in this study included, themonosaccharides glucose and glycerol (medium IM22, 248P and 153, 153m,153+Glycerol, 153m+Glycerol), the disaccharide; sucrose (medium SI and IM25)and the polysaccharide; dextrin (medium DEX). Antibacterial producingenvironmental actinobacteria, identified from earlier experiments on solid agar,showed that the liquid media DEX, IM25 and medium 153 were suitable for SMscreening for individual isolates.Dextrin proved to be the most appropriate carbon source for eliciting higherproduction of secondary metabolites in the selected isolates evaluated. This isconsistent with methodologies utilizing dextrin to increase the antibiotic productivityin actinobacteria by inducing synthases involved in SM production (Chatterjee &Vining, 1981), in contrast to this observation dextrin used in conjunction with anothersource of carbon yeast extract which decreased antibiotic production (Benslimane etal., 1995). In order to verify the positive effect of dextrin in fermentations, moreextensive studies need to be conducted using defined media incorporating statisticaldesign using stepwise discriminant analysis (SDA) in identifying substrates havingweak effects and those possessing strong effects in relation to secondary metaboliteproducing soil actinobacteria (Huck et al., 1991), or conducting response surfaceexperimental designs (Bull et al., 1990). Glycerol usually used as a alternative carbonsource to glucose, at a concentration of 20% w/v was substituted for glucose incomplex liquid fermentation media 153 and 153m, this resulted in suppressive effectson SM production. The same effect was shown by Gouveia et al. (2001) usingcomplex medium that high concentrations (20% w/v) of glycerol suppressedantibiotic production. The defined medium SI, containing only sucrose as a carbonsource was a poor fermentation medium, however another medium containing sucrosewith organic sources of carbon such as IM25 proved to be suitable for SM screening.It has been shown that by incorporating sucrose in complex media osmotic balance ismaintained at equilibrium between the cell and the external environment, so thatbioconversion of SM efficiently occurs (Elibol & Mavituna, 1998).4.2.2.2 Nitrogen sourcesSlowly utilizable organic nitrogen sources such as soybean meal (SBM) have beenshown to benefit antibiotic fermentations, due to the avoidance of immediate_____________________________________________________________________122