show Abstracthide AbstractST131 is one of the most common E. coli lineages found in clinical specimens globally. It is frequently associated with antimicrobial resistance to several drug classes, including to fluoroquinolones, aminoglycosides and extended-spectrum cephalosporins. Resistance to extended-spectrum cephalosporins is often attributable to the presence of CTX-M genes encoding extended spectrum beta-lactamases. This study used whole genome sequencing to characterize the evolutionary history of the lineage as it has emerged and spread worldwide, and to explain the genetic context of the CTX-M genes. In order to do this, several DH10B E. coli isolates were transformed with extracted CTX-M plasmids, then sequenced, enabling an approximate reconstruction of the plasmid sequences by bioinformatically removing DH10B-associated reads and undertaking de novo assembly of the residual data.