Porsch, E. A. | bioRxiv Science

bioRxiv2022

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.

microbiology↗

bioRxiv2022

Surface anchoring of the Kingella kingae galactan is dependent on the lipopolysaccharide O-antigen

Kingella kingae is a leading cause of bone and joint infections and other invasive diseases in young children. A key K. kingae virulence determinant is a secreted exopolysaccharide that mediates resistance to serum complement and neutrophils and is required for full pathogenicity. The K. kingae exopolysaccharide is a galactofuranose homopolymer called galactan and is encoded by the pamABC genes in the pamABCDE locus. In this study, we sought to define the mechanism by which galactan is tethered on the bacterial surface, a prerequisite for mediating evasion of host immune mechanisms. We found that the pamD and pamE genes are glycosyltransferases and are required for synthesis of an atypical lipopolysaccharide (LPS) O-antigen. The LPS O-antigen in turn is required for anchoring of galactan, a novel mechanism for association of an exopolysaccharide with the bacterial surface. SignificanceKingella kingae is an emerging pediatric pathogen and produces invasive disease by colonizing the oropharynx, invading the bloodstream, and disseminating to distant sites. This organism produces a uniquely multifunctional exopolysaccharide called galactan that is critical for virulence and promotes intravascular survival by mediating resistance to serum and neutrophils. In this study, we established that at least some galactan is anchored to the bacterial surface via a novel structural interaction with an atypical lipopolysaccharide O-antigen. Additionally, we demonstrated that the atypical O-antigen is synthesized by the pamD and pamE genes, located downstream of the gene cluster responsible for galactan biosynthesis. This work addresses how the K. kingae exopolysaccharide can mediate innate immune resistance and advances understanding of bacterial exopolysaccharides and lipopolysaccharides.

microbiology↗