A 14th century CE Brucella melitensis genome and the recent expansion of the Western Mediterranean clade

Abstract

Brucellosis is a disease caused by the bacteriumBrucellaand typically transmitted through contact with infected ruminants. It is one of the most common chronic zoonotic diseases and of particular interest to public health agencies. Despite its well-known transmission history and characteristic symptoms, we lack a more complete understanding of the evolutionary history of its best-known species—Brucella melitensis. To address this knowledge gap we fortuitously found, sequenced and assembled a high-quality ancientB. melitensisdraft genome from the kidney stone of a 14th-century Italian friar. The ancient strain contained fewer core genes than modernB. melitensisisolates, carried a complete complement of virulence genes, and did not contain any indication of significant antimicrobial resistances. The ancientB. melitensisgenome fell as a basal sister lineage to a subgroup ofB. melitensisstrains within the Western Mediterranean phylogenetic group, with a short branch length indicative of its earlier sampling time, along with a similar gene content. By calibrating the molecular clock we suggest that the speciation event betweenB. melitensisandB. abortusis contemporaneous with the estimated time frame for the domestication of both sheep and goats. These results confirm the existence of the Western Mediterranean clade as a separate group in the 14thCE and suggest that its divergence was due to human and ruminant co-migration.