Detection and Expression of Biosynthetic Genes in Actinobacteria ...

BERVANAKIS, G.Chapter 1: INTRODUCTION1.7.2.5 pH EffectsThe pH of the medium affects the growth rate, mycelial morphology and secondarymetabolism in actinobacteria (Braun & Vecht-Lifshitz, 1991). Actinobacteriapossesses a wide pH optimum for growth (usually between 6 and 8) while secondarymetabolism can only tolerate a narrow range within 0.2 pH units (James et al, 1991;Chen et al., 1999). Studies by Hayakawa et al. (1995), indicate that by increasing thepH value from 5.5 to 7.5 members of the rare actinobacterial genus Microbispora spp.displayed increased antimicrobial activity.Intracellular pH (pH i ) of most microorganisms is maintained near neutrality,independent of medium pH. However, as the hydrogen ion gradient across thecytoplasmic membrane increases, the cell is forced to direct its resources towardsmaintaining the desired intracellular pH, possibly diverting energy away from SMbiosynthesis (Forage et al., 1985). Consequently, Corvini et al. (2000) showed thatpH i is a important parameter in establishing optimal conditions for the excretion ofsecondary metabolites.1.7.2.6 Dissolved Oxygen (DOC)Secondary metabolite producing actinobacteria are obligate aerobes, thus providingcells with adequate supplies of oxygen is critical for respiration to proceed effectively(Liefke et al., 1990). A major obstacle in submerged fermentations is the difficultiesin delivering sufficient oxygen to bacterial cells due to its low solubility in aqueousmedia and limitations in gas-liquid mass transfer (Dick et al., 1994; El-Enshasy et al.,2000). However, a novel approach such as that use by Elibol and Mavituna (1997)making use of perfluorocarbons which are oxygen carriers and have a oxygensolubility of 10-20 times higher than that of water, lead to increases in antibioticyields.The major effect that oxygen has on SM biosynthesis is that it can induce or repressenzyme systems which catalyse the incorporation of oxygen into organic molecules(Forage et al., 1985; Yegneswaran & Gray, 1991; Stanbury et al., 1995). Kaiser et al.(1994) showed that by increasing the DOC during production of the antibioticmanumycin, they obtained an increase in the yield of manumycin as well as detectednew metabolites which were manumycin derivatives. Furthermore, Pfefferle et al.(2000) showed increased SM production in Streptosporangium strains by using_____________________________________________________________________44