Permeability issues in whole-cell bioprocesses and cellular membrane engineering

Nutrient uptake and waste excretion are among the many important functions of the cellular membrane. While permitting nutrients into the cell, the cellular membrane system evolves to guide against noxious agents present in the environment from entering the intracellular milieu. The semipermeable nature of the membrane is at odds with biomolecular engineers in their endeavor of using microbes as cell factory. The cellular membrane often retards the entry of substrate into the cellular systems and prevents the product from being released from the cellular system for an easy recovery. Consequently, productivities of whole-cell bioprocesses such as biocatalysis, fermentation, and bioremediations are severely compromised. For example, the rate of whole-cell biocatalysis is usually 1-2 orders of magnitude slower than that of the isolated enzymes. When product export cannot keep pace with the production rate, intracellular product accumulation quickly leads to a halt of production due to product inhibition. While permeabilization via chemical or physical treatment of cell membrane is effective in small-scale process, large-scale implementation is problematic. Molecular engineering approach recently emerged as a much better alternative. Armed with increasingly sophisticated tools, biomolecular engineers are following nature's ingenuity to derive satisfactory solutions to the permeability problem. This review highlights these exciting molecular engineering achievements.