Acetate assimilation system encoded by acs-yjcH-actP operon was required for APEC-induced cytotoxicity to macrophages. A To investigate the cytotoxic effect of acs, actP or satP mutants on HD11 macrophages. Cells culture supernatant after 2 hpi, 4 hpi, 8 hpi, and 12 hpi was collected to determine the LDH releases by HD11 cells. Cytotoxicity level was counted as a percentage of the amount of LDH released from infected macrophages relative to that of uninfected cells lysed with 10% Triton X-100. B To determine the cytotoxic effect of acs, actP or satP mutants on RAW264.7 macrophages. Data acquired from at least four independent experiments performed in triplicate, and the mean values ± the standard errors were shown. Statistical differences were determined using two-way ANOVA analysis (*P < 0.01).