Iron-Tannin Coating Reduces Clearance and Increases Tumor Colonization of Systemically Delivered Bacteria

Engineered bacteria offer a novel approach to targeted cancer therapy, but challenges remain in delivering enough bacteria safely for effective treatment. Previous efforts have used either a native or synthetic coating to achieve better control over the half-life of bacteria in the body but have limitations in delivery or versatility. In this work, we optimized and evaluated a synthetic coating for probiotic <i>Escherichia coli</i> Nissle 1917 to increase its half-life in blood and thereby increase the bioavailability of intravenous doses of bacteria to colonize and treat tumors. Using a simple one-pot chemical process, we coated bacteria with iron and tannic acid (FeTA) to form a temporary adhesive protective coating surrounding the bacterial cell surface. The iron to tannic acid ratio of the coating was optimized for intravenous use, and FeTA-coated bacteria of several different genetic backgrounds showed 15-fold higher survival in blood survival assays for up to 4 hours. We found that the FeTA coating reduced both complement-mediated bacterial killing and phagocyte-mediated bacterial killing <i>in vitro</i>. As a result, systemic delivery of attenuated bacteria had up to 60% colonization efficiency of FeTA-coated bacteria in an orthotopic breast cancer mouse model compared to 10% for the non-coated control, all the while maintaining a two-fold decrease in weight loss of attenuated bacteria compared to wild-type. Altogether, we show that an optimized FeTA coating significantly extends the half-life and colonization efficiency of engineered bacteria, overcoming a key limitation of their application in cancer therapy.