Analysis and effect of conventional flasks in shaking culture of Escherichia coli

Diffusion-driven fed-batch fermentation in perforated ring flasks

PURPOSE

Simultaneous membrane-based feeding and monitoring of the oxygen transfer rate shall be introduced to the newly established perforated ring flask, which consists of a cylindrical glass flask with an additional perforated inner glass ring, for rapid bioprocess development.

METHODS

A 3D-printed adapter was constructed to enable monitoring of the oxygen transfer rate in the perforated ring flasks. Escherichia coli experiments in batch were performed to validate the adapter. Fed-batch experiments with different diffusion rates and feed solutions were performed.

RESULTS

The adapter and the performed experiments allowed a direct comparison of the perforated ring flasks with Erlenmeyer flasks. In batch cultivations, maximum oxygen transfer capacities of 80 mmol L-1 h-1 were reached with perforated ring flasks, corresponding to a 3.5 times higher capacity than in Erlenmeyer flasks. Fed-batch experiments with a feed reservoir concentration of 500 g glucose L-1 were successfully conducted. Based on the oxygen transfer rate, an ammonium limitation could be observed. By adding 40 g ammonium sulfate L-1 to the feed reservoir, the limitation could be prevented.

CONCLUSION

The membrane-based feeding, an online monitoring technique, and the perforated ring flask were successfully combined and offer a new and promising tool for screening and process development in biotechnology.

A forced aeration system for microbial culture of multiple shaken vessels suppresses volatilization

Shake-flask culture, an aerobic submerged culture, has been used in various applications involving cell cultivation. However, it is not designed for forced aeration. Hence, this study aimed to develop a small-scale submerged shaking culture system enabling forced aeration into the medium. A forced aeration control system for multiple vessels allows shaking, suppresses volatilization, and is attachable externally to existing shaking tables. Using a specially developed plug, medium volatilization was reduced to less than 10%, even after 45 h of continuous aeration (~ 60 mL/min of dry air) in a 50 mL working volume. Escherichia coli IFO3301 cultivation with aeration was completed within a shorter period than that without aeration, with a 35% reduction in the time-to-reach maximum bacterial concentration (26.5 g-dry cell/L) and a 1.25-fold increase in maximum concentration. The maximum bacterial concentration achieved with aeration was identical to that obtained using the Erlenmeyer flask, with a 65% reduction in the time required to reach it.

Heterologous Expression of Toxic White Spot Syndrome Virus (WSSV) Protein in Eengineered Escherichia coli Strains

Aquacultural shrimps suffer economic lost due to the white spot syndrome virus (WSSV) that is the most notorious virus for its fatality and contagion, leading to a 100% death rate on infected shrimps within 7 days. However, the infection of mechanism remains a mystery and crucial problem. To elucidate the pathogenesis of WSSV, a high abundance of protein is required to identify and characterize its functions. Therefore, the optimal WSSV355 overexpression was explored in engineered Escherichia coli strains, in particular C43(DE3) as a toxic tolerance strain remedied 40% of cell growth from BL21(DE3). Meanwhile, a trace amount of WSSV355 was observed in both strains. To optimize the codon of WSSV355 using codon adaption index (CAI), an overexpression was observed with 1.32 mg/mL in C43(DE3), while the biomass was decreased by 35%. Subsequently, the co-expression with pRARE boosted the target protein up to 1.93 mg/mL. Finally, by scaling up production of WSSV355 in the fermenter with sufficient oxygen supplied, the biomass and total and soluble protein were enhanced 67.6%, 44.9%, and 7.8% compared with that in flask condition. Herein, the current approach provides efficacious solutions to produce toxic proteins via codon usage, strain selection, and processing optimization by alleviating the burden and boosting protein production in E. coli.

Control of carbon dioxide concentration in headspace of multiple flasks using both non-electric bellows pump and shaking incubator

Current methods of controlling gas in the headspace involve constant speed aeration and proportional-integral-differential (PID) controlled aeration using improved monitoring devices or gas cylinders. However, these approaches are restricted and inconvenient to use. In this study, we propose a method to control the CO₂ concentration in the headspace while maintaining the convenience of shake-flask culture. A combination of a non-electric bellows pump for shake-flask (NeBP-sf) and a CO₂ incubator was used to control the flask gas phase by shaking without additional external power. The CO₂ half-life, as an indicator of the ventilation ability of the system, was measured using a circulation direct monitoring and sampling system, and the NeBP-sf was optimised. The ventilation capacity varied depending on the shaking speed, and under optimal conditions, was 10 min compared with 45 min when only a breathable culture plug was used. In conventional microbial shaking culture, the CO₂ concentration in the flask gas phase remained higher than the 5% set-value with a maximum of 9%, resulting in a large concentration difference with the set point. Therefore, the ventilation capacity of the conventional shake-flask culture was insufficient for aerobic culture. Cultivation of Escherichia coli and Lactiplantibacillus plantarum using the system showed no significant difference between the set point and real point values. Thus, the system combined an NeBP-sf and a gas incubator built-in shaking table to achieve the reproducibility of gas control while maintaining a high level of convenience.

Bioprocess optimization of glutathione production by Saccharomyces boulardii: biochemical characterization of glutathione peroxidase

The well-known probiotic GRAS Saccharomyces boulardii (CNCM I-745) was used for the first time to produce glutathione (GSH). The culture conditions affecting GSH biosynthesis were screened using a Plackett-Burman design (PBD). Analyzing the regression coefficients for 12 tested variables, yeast extract, glucose, peptone, cysteine, temperature and agitation rate had a positive significant effect on GSH production with a maximum yeild 192 mg/L. The impact of kinetics of adding cysteine was investigated in 19 experiments during the growth time course (0-36 h), and the maximum yield of glutathione (235 mg/L) was obtained by addition of cysteine after 8 h post-inoculation. The most significant variables were further explored at five levels using central composite rotatable design (CCRD), giving a maximum production of GSH (552 mg/L). Using baffled flasks, the yield of GSH was increased to 730 mg/L, i.e., 1.32-fold increment. The two rate-limiting genes of GSH biosynthesis "γ-glutamyl cysteine synthetase (GSH1) and GSH-synthetase (GSH2)" were amplified and sequenced to validate the GSH biosynthetic potency of S. boulardii. The sequences of genes showed 99% similarity with GSH1 and GSH2 genes of S. cerevisiae. Glutathione peroxidase was purified and characterized from S. boulardii with molecular mass and subunit structure of 80 kDa and 35 kDa as revealed from native and SDS-PAGE, ensuring its homodimeric identity. The activity of GPx was reduced by 2.5-fold upon demetallization confirming its metalloproteinic identity. The GPx was strongly inhibited by hydroxylamine and DTNB, ensuring the implication of surface lysine and cysteine residues on the enzyme active site domains.

Analysis of porous breathable stopper and development of PID control for gas phase during shake-flask culture with microorganisms

We evaluated the ventilation ability of two types (plug-type and cap-type) of culture-stoppers having standard air permeability. The culture-stoppers were evaluated using the circulation direct monitoring and sampling system with CO₂ concentration in the gas phase of a shake-flask culture as an index. The half-lives of CO₂ in the headspace of the shake flask with the plug-type and cap-type stoppers were about 51.5 min and about 30.3 min, respectively. Based on these half-lives, we formulated a model equation to simulate the behaviour of CO₂ with different culture-stoppers. After validating the model equation by shake-flask culture with Saccharomyces cerevisiae, we investigated the effect of different ventilation abilities of the culture-stoppers on the growth of Pelomonas saccharophila and Escherichia coli: the sensitivity of the culture-stopper to the ventilation ability was dependent on the microorganism species. In the case of P. saccharophila, when the plug-type culture-stopper was combined with controlled CO₂ concentration (6%) in the flask, the maximum yield increased by twofold compared to that of the control. This study shows the importance of ventilation in headspace and conventional culture-stoppers during the shake-flask culture of microorganisms. The problems that may occur between the conventional shake-flask culture approach using a breathable culture-stopper and the next-generation shake-flask culture without a conventional culture-stopper were clarified from the evaluation of gas-permeable culture-stoppers. The importance of controlled gaseous phase in the headspace during shake-flask culture of the microorganisms was also elucidated. KEY POINTS: • Ventilation capacity of culture-stoppers was evaluated using the CO₂ half-life concentration. • Behaviour of microorganisms varies with the type of culture-stopper. • Developed a PID system for control of CO₂ in flask gas phase to enhance the shake-flask culture.