Function-preserving biointerface engineering of bacterial microswimmers for active cargo delivery

Surface engineering of bacterial microswimmers offers new opportunities for active cargo delivery by combining synthetic interfaces with intrinsic bacterial motility and environmental responsiveness. However, current strategies mainly focus on improving bacterial protection and cargo loading, whereas the effects of engineered interfaces on the biological functions required for active delivery remain poorly understood. Here, we developed a function-preserving biointerface engineering strategy by constructing thickness-tunable Fe^III-tannic acid (Fe^IIITA) coatings with approximate thicknesses of 10-70 nm on Escherichia coli. The effects of interfacial shielding on bacterial viability, motility, and chemotactic responsiveness were systematically investigated. Increasing coating thickness enhanced bacterial resistance to antibiotics and enzymatic degradation but progressively impaired swimming performance and chemotactic migration due to physical restriction. Importantly, removal of the excessive coating restored bacterial motility, confirming that functional impairment resulted from reversible interface-induced constraints rather than bacterial damage. A coating approximately 20 nm thick provided the optimal functional balance, retaining 82.3% of the swimming velocity of bare bacteria and preserving glucose-responsive chemotaxis. The optimized bacterial microswimmer further enhanced doxorubicin delivery relative to the nonmotile control and, under near-infrared irradiation, markedly suppressed tumor growth in vivo without evident systemic toxicity. This study establishes function preservation as a key principle for biointerface engineering of bacterial carriers and provides a strategy for developing active delivery systems with maintained biological functionality.