Bacillus anthracis evolutionary history: taking advantage of the topology of the phylogenetic tree and of human history to propose dating points

Abstract

This review expands on a talk I gave during the Biology of Anthrax meeting held in Bari, Italy (September 3 rd-6 th , 2019). The talk was a synthesis of recent investigations taking advantage of the topology of the Bacillus anthracis phylogenetic tree to propose tentative dating points and scenarios. Currently available whole-genome sequence (WGS) data allowed identifying single nucleotide polymorphisms (SNPs) among B. anthracis strains and drawing phylogenetic trees. The geographic origin of the strains and the topology of the tree were used to infer spreading events. Five star-like patterns in the tree (polytomies), each containing at least six branches, were detected. The analysis of the geographic distribution of the strains constituting one such polytomy suggests that it emerged not more than a few centuries ago. The key observation allowing this dating is the finding that the polytomy is anchored into Western Europe and that the main North-American lineage emerged from one of its branches, indicative of a post-Columbian export. From this point, I propose additional working hypotheses which may allow dating key nodes along the phylogeny of B. anthracis corresponding to four Out-of-Africa events. While trade of contaminated animal products seems to be the predominant driving force underlying modern long-distance spreading of B. anthracis, invasive military operations and more generally borders instabilities may have played an important role in earlier times. The testing of these hypotheses will require the sequencing of a significant number of additional strains from many countries.