LuxS/AI-2 system facilitate the dissemination of antibiotic-resistant plasmids in Klebsiella pneumoniae.

Plasmid conjugation is a central mechanism driving the dissemination of antibiotic resistance in Klebsiella pneumoniae. However, the conjugative operon requires specific stimuli for activation. Identifying signals and elucidating the underlying mechanisms is crucial in combating plasmid spread. This study uncovers a key mechanism promoting the dissemination of high-risk plasmids, including IncFII, IncX3, and IncX4 types, in K. pneumoniae. We observed that increased donor density significantly enhanced conjugation, and transcript levels of both conjugation and AI-2 quorum sensing genes were markedly upregulated. By mutating the luxS and lsrR genes in K. pneumoniae 1678, we found that plasmid conjugation efficiency decreased in the 1678ΔluxS mutant, while it significantly increased in the 1678ΔlsrR mutant. RT-qPCR and β-galactosidase assays demonstrated that LsrR represses transcription of relaxosome and T4CP genes, while AI-2 (synthesized by LuxS) activates their expression. AlphaFold3 docking models suggest LsrR may bind directly to IncX plasmid relaxase promoters, inhibiting their expression. Adding external AI-2 signals revealed no effect on plasmid conjugation when LsrR was absent, confirming the dependence of AI-2 signals on LsrR repression. In conclusion, AI-2-mediated signaling enhances donor density effects on plasmid conjugation by de-repressing LsrR-mediated suppression.