Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 1: INTRODUCTIONstudies. The most common SM production methods involve liquid (submerged) andsolid-state fermentations. In the following paragraphs a description is given of thefermentation conditions and their influences in secondary metabolite production byactinobacteria.1.7.1 Physical Conditions Affecting Secondary Metabolite ProductionOf primary consideration in submerged fermentation of secondary metabolitesinvolving filamentous microorganisms such as fungi and actinobacteria is thefermentation vessel (Whitaker, 1992). For actinobacteria shake flasks are commonlyused with angular baffles, these indentations aid in the efficient mixing and inincreased oxygen transfer (Katzer et al., 2001). Adequate oxygen supply is one of themost frequent problems associated with the use of shake flasks (Büchs, 2001).1.7.2 Submerged Fermentation Conditions Affecting SecondaryMetabolite ProductionThe expression of secondary metabolism in actinobacteria is dependent on a numberof biochemical, physical and environmental parameters (Table 10). Additives to liquidmedia which can be broadly applied to elicit or enhance the production of a widerange of secondary metabolites is not known. However, indications are thatcompounds that limit the release of easily assimilatable nutrients or trap inhibitorysubstances promote the enhancement and induction of novel secondary metabolites bytrapping inhibitory substances. These compounds include natural zeolites such asmagnesium phosphate which form complexes with ammonium salts (Shapiro, 1989;Masuma et al., 1983). Similarly, it has been established that organic nitrogen sources,such as soy-based media can be used to induce the expression of certain classes ofsecondary metabolites in different actinobacteria (Hessler et al., 1997; Gouveia et al.,1999).Table 10. Factors affecting secondary metabolismMedium CompositionFermentation ConditionsCarbon sourcepHNitrogen sourceTemperatureInorganic phosphateOxygen TransferInorganic saltsCarbon DioxideTrace metalsAgitationPrecursorsOxidation-Reduction Potential (E h )InhibitorsRheologyInducersOsmotic PressureIonic StrengthFermentation Vessel_____________________________________________________________________37