The sensitivity of R. anatipestifer CH-1 and its derived strains to non-inactivated duck serum. The non-inactivated duck serum and inactivated duck serum were obtained by centrifuging from the jugular vein-collected blood of 7-day-old ducklings as described in the “Materials and methods”. R. anatipestifer CH-1pLMF03, R. anatipestifer CH-1ΔfurpLMF03 and R. anatipestifer CH-1ΔfurpLMF03::fur were grown in GCB liquid medium at 37 °C in a shaking incubator to exponential phase (OD600 = 1.0–1.5). Cells were collected and diluted in PBS to 109 CFU/mL, and the bacterial suspensions were incubated with 50% non-inactivated duck serum (A) or 50% inactivated duck serum (B) for 0.5 h or 1 h at 37 °C. Bacterial survival was enumerated by plating and counting colonies the following day as described in the “Materials and methods”. The error bars represent the standard deviations of three independent experiments and three replicate samples for each experiment. Statistical significance was determined using two-way ANOVA (****P < 0.0001).