Effect of aeration and pH on gramicidin S production by Bacillus brevis

Abstract

Although the biosynthesis of the antibiotic gramicidin S (GS) by Bacillus brevis ATCC 9999 has been studied extensively, almost no attention has been given to environmental control of its fermentation process. In this respect, GS fermentations conducted in a 7.5 dm3 fermentor in complex (YP) medium revealed that a high aeration rate resulted in a high biomass yield (12 g DCW dm−3) with very low GS levels (170 mg GS dm−3). Lowering the aeration rate (5 dm3 air min−1 at 300 rev min−1) caused a dramatic increase in GS formation (2100 mg GS dm−3) and comparable but slower biomass formation. In chemically‐defined (F3/6) medium fermentations, an aeration rate of 5 dm3 air min−1 at 300 rev min−1 was apparently too high as only 0.104 mg GS mg−1 DCW was produced. A much lower aeration rate (2 dm3 air min−1 at 250 rev min−1) was needed to arrive at a higher specific antibiotic level: 0.130 mg GS mg−1 DCW. These data seem compatible with the finding that oxygen is known to inactivate the GS‐synthetases. Furthermore, keeping the pH constant at 7.3 under low aeration conditions increased specific GS production up to 0.220 mg GS mg−1 DCW in YP, as well as in F3/6 fermentations. Both environmental pH and dissolved oxygen tension clearly affect growth pattern, growth extent and GS production in these high yielding media. These data stress the importance of controlling pH and aeration rate during GS fermentations.