Inducible Escherichia coli fermentation for increased plasmid DNA production

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Abstract
Bacterial plasmids are the vectors of choice for DNA vaccines and short‐term gene therapeutics. Growing plasmid DNA by microbial (Escherichia coli) fermentation is usually combined with alkaline lysis/chromatography methods of purification. To date, typical plasmid fermentation media and processes result in yields of 100–250 mg of plasmid DNA/l of culture medium, using standard high‐copy pUC origin‐containing plasmids. In order to address this initial and yield‐limiting upstream step, we identified novel fermentation control parameters for fed‐batch fermentation. The resulting fermentation strategies significantly increased specific plasmid yield with respect to cell mass while enhancing plasmid integrity and maintaining supercoiled DNA content. Fed‐batch fermentation yield exceeding 1000 mg of plasmid DNA/l was obtained after reduction of plasmid‐mediated metabolic burden during growth, and yields up to 1500 mg of plasmid DNA/l have been achieved with optimized plasmid backbones. Interestingly, by inducing high plasmid levels after sufficient biomass accumulation at low temperature and restricted growth, cells were able to tolerate significantly higher plasmid quantities than cells grown by conventional processes. This 5–10‐fold increase in plasmid yield dramatically decreases plasmid manufacturing costs and improves the effectiveness of downstream purification by reducing the fraction of impurities.