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.

Genetic diversity of Francisella tularensis subsp. holarctica in Kazakhstan

“…A third reason is the poor knowledge of F. tularensis closest neighbors including F. novicida. It is the knowledge of closest neighbors which provided the strongest argument for an African origin of both Mycobacterium tuberculosis and B. anthracis [56,57].…”

Sequence Variability of pXO1-Located Pathogenicity Genes of Bacillus anthracis Natural Strains of Different Geographic Origin

“…These animals could be sick with anthrax, and goods could be contaminated with B. anthracis spores, which are easily transported and remain viable. Perhaps the anthropogenic transportation of spores was the main driver of the spread of B. anthracis across the planet [14]. Due to the natural and anthropogenic transfer, genetically related strains may be introduced into distant regions, where their further evolution may proceed differently.…”

A Review of Arguments for the Existence of Latent Infections of Bacillus anthracis, and Research Needed to Understand Their Role in the Outbreaks of Anthrax

“…Vergnaud's presentation was on the spread of B. anthracis lineages throughout Eurasia [26]. The implication was that military operations have played a major role in the spread of B. anthracis in agreement with the inclusion of anthrax among war-associated diseases [27].…”

“…The much more extensive and referenced discussions in these two documents are paraphrased in the present review. In particular, Turnbull's collective work [2] (pages [26][27], more than 10 years ago, linked a significant part of these anomalies to our modest understanding of a number of features regarding B. anthracis interaction with its hosts including prolonged incubation, carrier state, unapparent, chronic and latent infections. In the rest of this review, we will use the term "latent infection" to designate all of these categories (latent infections in the broad sense, sensu lato; sensu stricto would refer to the disease contained in an inactive state by the immune system).…”