Evolution of host adaptation in the Salmonella typhoid toxin.

Abstract

The evolution of virulence traits is central for the emergence or re-emergence of microbial pathogens and for their adaptation to a specific host1–5. Typhoid toxin is an essential virulence factor of the human-adapted bacterial pathogen Salmonella Typhi6,7, the cause of typhoid fever in humans8–12. Typhoid toxin has a unique A2B5 architecture with two covalently linked enzymatic “A” subunits, PltA and CdtB, associated to a homopentameric “B” subunit made up of PltB, which has binding specificity for N-acetylneuraminic acid (Neu5Ac) sialoglycans6,13 predominantly present in humans14. Here we examined the functional and structural relationship between typhoid toxin and ArtAB, an evolutionarily related AB5 toxin encoded by the broad-host Salmonella Typhimurium15. We found that ArtA and ArtB, homologs of PltA and PltB, can form a functional complex with the typhoid toxin CdtB subunit after substitution of a single amino acid in ArtA, while ArtB can form a functional complex with wild type PltA and CdtB. We also found that after addition of a single terminal Cys residue, a CdtB homolog from cytolethal distending toxin can form a functional complex with ArtA and ArtB. In line with the broad host specificity of S. Typhimurium, we found that ArtB binds human glycans, terminated in N-acetylneuraminic acid, as well as glycans terminated in N-glycolylneuraminic acid (Neu5Gc), which are expressed in most other mammals14. The atomic structure of ArtB bound to its receptor shows the presence of an additional glycan-binding site, which broadens its binding specificity. Despite equivalent toxicity in vitro, we found that the ArtB/PltA/CdtB chimeric toxin exhibits reduced lethality in an animal model, indicating that the host specialization of typhoid toxin has optimized its targeting mechanisms to the human host. This is a remarkable example of toxin evolution to broaden its enzymatic activities and adapt to a specific host.