Spanish Outbreak Isolates Bridge Phylogenies of European and American Bacillus anthracis

The geographical origin of a major present-day phylogenetic group (A branch WNA; A.Br.WNA) of American Bacillus anthracis is controversial. One hypothesis postulated that the anthrax pathogen reached North America via a then-existing land bridge from northeastern Asia thousands of years ago. A competing hypothesis suggested that B. anthracis was introduced to America a couple of hundred years ago, related to European colonization. The latter view is strongly supported by genomic analysis of a group of French B. anthracis isolates that are phylogenetically closely related to the North American strains of the A branch A.Br.WNA clade. In addition, three West African strains also belong to this relationship group. Recently, we have added a Spanish strain to these close relatives of the WNA lineage of American B. anthracis. Nevertheless, the diversity of Spanish B. anthracis remains largely unexplored, and phylogenetic links to European or American relatives are not well resolved. Here, we genome sequenced and characterized 29 new B. anthracis isolates (yielding 18 unique genotypes) from outbreaks in west central and central Spain in 2021. Applying comparative chromosomal analysis, we placed the chromosomes of these isolates within the established phylogeny of the A.Br.008/009 (A.Br.TEA) canonical SNP group. From this analysis, a new sub-clade, named A.Br.11/ESPc, emerged that constitutes a sister group of American A.Br.WNA.