Colonization in ducks. Groups of ducklings (n = 6/group) were inoculated orally with approximately 109 CFU of the P. multocida WT strain or each mutant strain. The blood (A), spleen (B), lung (C) and liver (D) were collected at 12 h or 24 h post-infection, and the bacterial loads of each strain were calculated as CFU/mL in the blood and CFU/g in the three tissues. The asterisk above the error bar indicates significance compared with the WT group. **, p < 0.01; ***, p < 0.001.