CpxR is essential for efficient colonization and virulence of APEC. A Determination of bacterial virulence. Seven-day-old ducks were infected with APEC strains, and the mortality was monitored until 7 days post-infection. Negative controls were injected with PBS. B–E Bacterial colonization, survival and competition in ducks. Seven-day-old ducks were infected with APEC strains in noncompetitive (B–D) and competitive (E) assays. Ducks were sacrificed at 24 h post-infection, and bacteria were recovered from the lungs, livers and spleens. The competitive indices were calculated and shown for the competitive assays (E). Nonparametric Mann–Whitney U-test was carried out to determine statistical significance (*P < 0.05).