Original data from animal experiments.

The global rise of multidrug-resistant Escherichia coli ST410 presents a growing clinical threat. We isolated a highly lytic phage targeting ST410, but rapid resistance emerged both in vitro and in vivo. Beyond receptor mutations, we identified a hypermucoid mutant that evades phage infection primarily through colanic acid overproduction, providing broad phage resistance. This colanic acid overproduction also led to diminished macrophage phagocytosis in vitro and accelerated mortality in mice. Transcriptomic and genetic analyses linked this phenotype to activation of the Rcs pathway. Notably, naturally occurring ST410 isolates with colanic acid-overproducing phenotypes were also observed, suggesting that capsule-mediated immune evasion may arise in clinical populations. To overcome resistance, we designed a rational four-phage cocktail with diverse receptors, achieving robust therapeutic efficacy in infection models. Our findings highlight that phage resistance can be accompanied by enhanced virulence potential through capsule-mediated immune evasion, with colanic acid playing a central role, emphasizing important considerations for phage therapy design.