Dual-functional quorum sensing signal synthases DspII and DspI coordinate virulence switch in Pseudomonas aeruginosa.

The transition between motility-associated acute infection and biofilm-associated chronic infection is crucial for bacterial proliferation and survival within hosts. This process of transition can be influenced by various host and environmental cues, however, whether bacteria could actively modulate the switch remains poorly understood. Here we identify a novel enoyl-CoA hydratase/isomerase, PA0744 (designated as DspII), which is co-upregulated with the previously characterized enoyl-CoA hydratase/isomerase DspI in a cell density-dependent manner to induce chronic-to-acute virulence switch in Pseudomonas aeruginosa. The two proteins form a heterocomplex that synthesizes the DSF-family quorum sensing signal cis-2-decenoic acid (CDA), which positively regulates the expression of the phosphodiesterase RbdA, and subsequently, activates biofilm dispersion and swarming motility through the c-di-GMP/FleQ pathway. Intriguingly, the DspII-DspI complex also synergistically antagonizes the GacA-upregulated sRNAs, RsmY and RsmZ, leading to overproduction of type III secretion system (T3SS) and augment of acute pathogenesis. Notably, the affinity of DspII-DspI interaction and consequently T3SS gene expression are fine-tuned by CDA levels. This study unveils the novel enzymatic role of the DspII-DspI complex in CDA biosynthesis as well as its non-enzymatic role in T3SS gene regulation, which presents a simple yet sophisticated bacterial signal-driven auto-regulatory mechanism that governs the chronic-to-acute virulence switch in P. aeruginosa.