Genetic boundaries to delineate the typhoid agent and other Salmonella serotypes into distinct natural lineages

- S. typhi for the first time was demonstrated to be a natural cluster of bacteria, equivalent to a biological species.
- S. typhi is circumscribed by a clear-cut genetic boundary from other bacteria as closely related as S. typhimurium.
- The genetic boundary to delineate Salmonella to discrete genetic clusters is digital and may be applied to other bacteria.

The deadly human typhoid agent was initially classified as a species called Salmonella typhi but later reclassified as a serovar of Salmonella enterica together with other pathogenically diverse serovars. The dynamic changes of Salmonella taxonomy reflect the need to clarify the phylogenetic status of the Salmonella serovars: are they discrete lineages or variants of a genetic lineage? To answer this question, we compared S. typhi and other Salmonella serotypes. We found that the S. typhi and Salmonella typhimurium strains had over 90% and ca. 80%, respectively, of their genes identical; however, between S. typhi and S. typhimurium, this percentage dropped to 6%, suggesting the existence of genetic boundaries between them. We conclude that S. typhi and the other compared Salmonella serovars have developed into distinct lineages circumscribed by the genetic boundary. This concept and methods may be used to delineate other Salmonella serotypes, many of which are polyphyletic, needing differentiation.