The acetate assimilation system encoded by acs-yjcH-actP operon contributed to APEC virulence and phenotype fitness. A The mortality rates of wild-type FY26 and variants were determined by duck model to investigate the effect of acs-yjcH-actP deletion on the virulence of APEC. Survival rate was measured after 7 days post-infection. The two-way ANOVA was performed for survival assays (*P < 0.01). B Determining the effect of acs-yjcH-actP operon deletion on APEC colonization in vivo. Systemic infection experiment of the duck infection model was performed to assess the bacteria proliferation in duckling organs and blood at 24 hpi. Statistical significances were identified using a nonparametric Mann–Whitney U test (*P < 0.01). C The mortality rates of wild-type FY26, the mutant FY26Δacs-yjcH-actP, and the complemented FY26Cacs-yjcH-actP were determined by chick model. D Systemic infection experiment of the duck infection model was performed to assess the bacteria proliferation in chick organs and blood at 24 hpi. E Histopathology of lungs from ducks infected intratracheally with wild-type FY26, FY26Δacs, FY26Δacs-yjcH-actP, FY26Cacs-yjcH-actP, and the negative control PBS. Duck lungs at 12 hpi, 24 hpi, 36 hpi, or 48 hpi were fixed, and sections were stained with HE. The pathological changes of the lungs were observed under a light microscope. a–d The image observation of lung sections from PBS-inoculated ducks at 12 hpi, 24 hpi, 36 hpi, or 48 hpi, respectively. e–h The lesions of lung sections from FY26-infected ducks at four time points. i–l The lesions of lung sections from FY26Δacs-infected ducks. m–p The lesions of lung sections from FY26Δacs-yjcH-actP-infected ducks. q–t The lesions of lung sections from FY26Cacs-yjcH-actP-infected ducks. Scale bars 100 μm. BV, blood vessel; PB, primary bronchus; SB, secondary bronchus; PL, parabronchial lumen; PA, pulmonary alveolus.