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(d(1) = 20 mm, d(i)/d(T) = 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 k(L)a 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, mu(max), was 0.68 +/- 0.01 h(-1) and the final biomass concentration obtained, X-final was 3.8 +/- 0.05 g L-1. Similarly for B. subtilis (1500 rpm, 1 vmm) was 0.45 +/- 0.01 h(-1) and X-final 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 mu(max) and X-final 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 k(L)a values. Very similar growth kinetics were observed at both scales giving mu(max) values of 0.94 and 0.97 h(-1), and X-final 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. (C) 2007 Elsevier B.V. All rights reserved.