Coupling of Oxygen Transfer Limitation, Substrate Gradients and Acetate Overflow in High-Cell-Density Fed-Batch Cultivation of Escherichia coli

High-cell-density fed-batch cultivation of Escherichia coli can achieve high volumetric productivity, but scale-up creates a growing mismatch between cellular demand and reactor transport capacity. This review integrates oxygen-transfer limitation, substrate gradients and acetate overflow within one multiscale framework. As biomass rises, oxygen uptake can approach the achievable oxygen transfer rate, while slower mixing and localized feeding expose circulating cells to repeated high-glucose/low-oxygen and low-glucose/higher-oxygen zones. These trajectories promote rapid switching between respiratory and fermentative states, redirect carbon toward acetate and reduce biomass or recombinant-product yield. Scale-down evidence shows that exposure amplitude, duration and frequency are more informative than bulk averages alone. The available control options therefore have complementary roles: reactor and gas-delivery design improve transfer, adaptive or distributed feeding limits local carbon excess, soft sensors and model-predictive control coordinate constrained actuators, and strain engineering can improve robustness to transient gradients. Linking computational fluid dynamics to cell trajectories and reduced metabolic models provides a practical basis for scale-down validation and digital-twin development. Acetate accumulation is best interpreted as a transport-metabolism mismatch rather than a single pathway defect, so reliable scale-up requires coordinated process, equipment and host design.