Acquisition, co-option, and duplication of the rtx toxin system and the emergence of virulence in Kingella

The Kingella genus includes two pathogenic species, namely K. kingae and K. negevensis, as well as strictly commensal species. Both K. kingae and K. negevensis secrete a toxin called RtxA that is absent in the commensal species. Phylogenetic analysis demonstrates that the toxin-encoding operon rtxCrtxAtolC was acquired by a common ancestor of the pathogenic Kingella species and that a preexisting type I secretion system was co-opted for toxin export. Subsequent genomic reorganization distributed the toxin machinery across two loci, with 30-35% of K. kingae strains containing two copies of the rtxA toxin gene. The rtxA duplication is largely clonal and strongly associated with invasive disease. In assays with isogenic strains, a single copy of rtxA was associated with reduced virulence in vitro. This study establishes the critical steps in the evolutionary transition from commensal to pathogen, including horizontal gene transfer, co-option of an existing secretion system, and gene duplication.