FIG 4.

The expression of cheA is regulated by ArcA, and cheA contributes to APEC virulence. (A) The expression of cheA and cheW is significantly downregulated in the arcA mutant in response to duck serum but not in response to LB under microaerobic (5% oxygen) conditions. Bacteria were grown in serum and LB under microaerobic conditions before being subjected to RNA extraction. qRT-PCR expression values are means plus standard deviations (error bars) from three independent experiments. Analysis of statistical significance was performed using two-way ANOVA (***, P < 0.01). (B) Deletion of cheA and cheW does not affect the growth of APEC XM in LB or duck serum under aerobic or microaerobic conditions. the OD₆₀₀ was monitored every 2 h. The experiments were performed at least three times in quadruplicate. (C) Deletion of cheA significantly attenuates the virulence of APEC XM. The cheA mutant, the cheA and cheW double deletion mutant, and the WT strain were inoculated into 10 7-day-old ducks using a dose of 1 × 105 CFU of per bird, and the mortality was monitored for 1 week. The virulence of the cheA mutant is significant attenuated compared to that of the WT strain (P < 0.05 by Fisher's exact test). Deletion of both cheA and cheW recovers virulence to the WT level.