ERP011535 PRJEB10304 ena-STUDY-PASTEUR-13-08-2015-10:21:11:816-216 ena-STUDY-PASTEUR-13-08-2015-10:21:11:816-216 Evolution and adaptation of the epidemic dysentery bacterium Shigella dysenteriae type 1 (Shiga’s bacillus) during the pre and post-antibiotic era Epidemics of dysentery with a high mortality rate have been linked to Shigella dysenteriae type 1 (Sd1), the historical Shiga’s bacillus. Over the last century, Sd1 has recurred in many outbreaks spanning over four continents where the circumstances of its emergence and regression still remain unknown. So far, we have collected and sequenced the whole genome of 234 isolates from epidemics and sporadic cases between 1915 and 2011 in order to establish an accurate population framework. Our phylogenetic analysis shows that the current Sd1 population descends from a common ancestor that existed around the year 1850 and which diversified into several lineages. We were also able to trace back the transmission routes of these lineages across the globe. This work shows that there are two genetic supports for its antibiotic resistance, the first being through the acquisition of different types of multi-resistance plasmids between 1969 and 1990; and then from the early 1980s due to the acquisition of the pathogenicity genomic island SRL-PAI. This study provides a unique insight into the Shigella dysenteriae‘s bacterial evolutionary factors, which allows us to better understand the factors of its sporadic reemergence. Global phylogeography and evolutionary history of Shigella dysenteriae type 1 Epidemics of dysentery with a high mortality rate have been linked to Shigella dysenteriae type 1 (Sd1), the historical Shiga’s bacillus. Over the last century, Sd1 has recurred in many outbreaks spanning over four continents where the circumstances of its emergence and regression still remain unknown. So far, we have collected and sequenced the whole genome of 234 isolates from epidemics and sporadic cases between 1915 and 2011 in order to establish an accurate population framework. Our phylogenetic analysis shows that the current Sd1 population descends from a common ancestor that existed around the year 1850 and which diversified into several lineages. We were also able to trace back the transmission routes of these lineages across the globe. This work shows that there are two genetic supports for its antibiotic resistance, the first being through the acquisition of different types of multi-resistance plasmids between 1969 and 1990; and then from the early 1980s due to the acquisition of the pathogenicity genomic island SRL-PAI. This study provides a unique insight into the Shigella dysenteriae‘s bacterial evolutionary factors, which allows us to better understand the factors of its sporadic reemergence.