Schematic diagram of CpxR-mediated regulation of T6SS2 expression in APEC. According to previous studies, many environmental stresses and surface adhesion sensed by the NlpE protein can activate the Cpx response [21, 22]. After activation, the CpxA protein phosphorylates the CpxR protein. The phosphorylation of CpxR enhances its binding to the T6SS2 hcp2B promoter and consequently upregulates transcription of the hcp2B operon. The upregulation of T6SS2 contributes to increased antibacterial competition, invasion and survival, which are required for the virulence of APEC.