Evolution of host adaptation in the Salmonella typhoid toxin

The evolution of virulence traits is central for the emergence or re-emergence of microbial pathogens and for their adaptation to a specific host(1–5). Typhoid toxin is an essential virulence factor of the human-adapted bacterial pathogen Salmonella Typhi(6,7), the cause of typhoid fever in humans(8–12). Typhoid toxin has a unique A(2)B(5) 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) sialoglycans(6,13) predominantly present in humans(14). Here we examined the functional and structural relationship between typhoid toxin and ArtAB, an evolutionarily related AB(5) toxin encoded by the broad-host Salmonella Typhimurium(15). 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 mammals(14). 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.