Design and characterisation of a miniature stirred bioreactor system for parallel microbial fermentations

The establishment of a high productivity microbial fermentation process requires the experimental investigation of many interacting variables. In order to speed up this procedure a novel miniature stirred bioreactor system is described which enables parallel operation of 4–16 independently controlled fermentations. Each miniature bioreactor is of standard geometry (100 mL maximum working volume) and is fitted with a magnetically driven six-blade miniature turbine impeller (di = 20 mm, di/dT = 1/3) operating in the range 100–2000 rpm. Aeration is achieved via a sintered sparger at flow rates in the range of 0–2 vvm. Continuous on-line monitoring of each bioreactor is possible using miniature pH, dissolved oxygen and temperature probes, while PC-based software enables independent bioreactor control and real-time visualisation of parameters monitored on-line. In addition, a new optical density probe is described that enables on-line estimation of biomass growth kinetics without the need for repeated sampling of individual bioreactors. Initial characterisation of the bioreactor involved quantification of the volumetric oxygen mass transfer coefficient as a function of agitation and aeration rates. The maximum kLa value obtained was 0.11 s−1. The reproducibility of E. coli TOP10 pQR239 and B. subtilis ATCC6633 fermentations was shown in four parallel fermentations of each organism. For E. coli (1000 rpm, 1 vvm) the maximum specific growth rate, μmax, was 0.68 ± 0.01 h−1 and the final biomass concentration obtained, Xfinal, was 3.8 ± 0.05 g L−1. Similarly for B. subtilis (1500 rpm, 1 vmm) μmax was 0.45 ± 0.01 h−1 and Xfinal was 9.0 ± 0.06 g L−1. Biomass growth kinetics increased with increases in agitation and aeration rates and the oxygen enrichment for control of DOT levels enabled μmax and Xfinal as high as 0.93 h−1 and 8.1 g L−1 respectively to be achieved. Preliminary, scale-up studies with E. coli in the miniature bioreactor (100 mL working volume) and a laboratory scale 2 L bioreactor (1.5 L working volume) were performed at matched kLa values. Very similar growth kinetics were observed at both scales giving μmax values of 0.94 and 0.97 h−1, and Xfinal values of 5.3 and 5.5 g L−1 respectively. The miniature bioreactor system described here thus provides a useful tool for the parallel evaluation and optimisation of microbial fermentation processes.