Since scientists found the way to use microorganisms for synthesis or conversion of chemical compounds, bioreactors have been evolving from the simple flasks to sophisticated machines with a lot of options for bioprocess control and monitoring. Bioreactors should support cells with all necessary nutrition, maintain an appropriate pH level and gas balance. Therefore, approaches for mass transfer are always a spotlight.
The most popular fermentation technologies that are currently presented on market use mechanical stirrers or compressed gas for energy input. Both technologies are not perfect. For example, mechanical stirrer promotes the formation of zones with high turbulence and stagnation levels, which leads to a heterogenic distribution of nutrients over the culture volume and non-uniform elimination of metabolites produced by cells during the fermentation process.
In addition, continuous motion of stirring blades causes shear stress and local overheating near their ends that also negatively influence on cultivating cells. In contrast, airlift bioreactors use air flow that promotes less traumatic for cells and more energy efficient cultivation process. However, airlift bioreactors have a high level of foaming which leads to the death of cultured cells on the surfaces of emerging bubbles. Moreover, they are limited in bioreactor volume usage and less effective for viscous liquids.
Inspired by vortex properties, engineers from the Center of Vortex Technologies (Novosibirsk, Russia) developed a radically different approach to the mass transfer and aeration that was implemented it in the construction of a gas-vortex bioreactor (European Patent № EP2746382 A1, 2011). The gas-vortex bioreactor uses top centrifugal activator and propeller to create a concentrated air vortex that promotes gas vortex movement in the liquid phase via stabilizing disk, located on the medium surface. Additionally, bioreactor has a porous sparger connected to a bottom air supplier that allows generating bubbles with a diameter less than 1–2 mm. This approach significantly increases the interfacial contact surface and results in three-dimensional movement of the liquid medium.
According to the Center of Vortex Technologies data, the gas vortex provides mild and efficient mixing of liquids (including viscous ones), prevents foaming during fermentation and allows the use of 10–90% of the reactor volume for the bioprocess. The absence of a mechanical stirrer eliminates problems related to it and, at the same time, reduces the energy consumption of the agitation system (0.3 kW/m3 for the gas-vortex bioreactor in comparison with 1–4 kW/m3 for mechanical and airlift bioreactors). It is assumed that the aeration-stirring system of the gas-vortex bioreactor may be universal for the cultivation of all cell types used in biotechnology (eukaryotes, prokaryotes, fungi).
Development Of Universal And Cost-efficient Cultivation Technologies
In this work, the efficiency of gas-vortex fermentation technologies for the cultivation of recombinant E. coli strains were evaluated in comparison with mechanical bioreactors and flasks on shakers.
One of the most crucial parameters for E.Coli cultivation is the oxygen transfer rate (OTR), which is described by the equation:
OTR = kLa(DO′−DO)
where kLa is the volumetric oxygen mass transfer coefficient, DO′ is the oxygen saturation concentration and DO is the oxygen concentration in the liquid phase. The volumetric oxygen mass transfer coefficient represents the efficiency of oxygen delivery to a bioreactor and directly depends on the total interfacial area. Using dynamic sulfite method, it was shown that the gas-vortex bioreactor had a 3.6 times higher kLa in comparison with the mechanical bioreactor (18 ± 2.8 H−1 vs 5 ± 0.1 H−1). Thus, the combination of the gas vortex and the porous sparger markedly enhances the oxygen solubility in the aqueous phase and could be used for the improvement of aerobic fermentation processes.
With uniform protocols for the cultivation of auxotrophic E. coli C600/pRT strain, that expresses Mu-MLV reverse transcriptase, it was shown that gas-vortex approach increases the yield of recombinant protein in 1.7 and 3.5 times in comparison with flasks and a mechanical bioreactor, respectively. Since auxotrophic strains are sensitive to mass transfer efficiency, the usage of gas-vortex bioreactors may be more beneficial for them.
Each industrial E.Coli producing strain demands thorough optimization of bioprocess parameters as medium pH, agitation speed, induction point, and inductor concentration. In this study, E.Coli BL21(DE3)/pFK2 strain, that expresses recombinant analog of lactaptin (human milk peptide with anticancer properties), was used for the comparative analysis of bioprocess optimization in gas-vortex and mechanical bioreactors. It was shown that the usage of the gas-vortex system allowed to enhance the productivity of recombinant lactaptin expression up to a twofold difference (from 24% to 54%) in comparison with the stirrer bioreactor.
The revealed mass transfer advantages of the gas-vortex approach along with the reduced power consumption (10–12 times lower than commonly used bioreactors, according to manufacturer’s specifications) allow proposing this type of bioreactors as a novel base platform for the subsequent development of universal and cost-efficient cultivation technologies.
This study, Analysis of the efficiency of recombinant E. coli strain cultivation in a gas-vortex bioreactor, was recently published by Anna V. Savelyeva, Anna A. Nemudraya, Vladimir F. Podgornyi, Vladimir A. Richter and co-workers in the journal Biotechnology and Applied Biochemistry.
