Genetic material that was acquired sporadically across the Central Asia clade within the Bhutanese isolates included the extended-spectrum β-lactamase (ESBL) gene blaCTXM-15, which was independently acquired by two S. sonnei on different incompatibility plasmids (IncK and IncI1), conferring resistance to third-generation cephalosporins (Fig. 2). The plasmid replication and transfer region of the IncK plasmid shared substantial sequence homology with that of the enteroaggregative E. coli IncI1 plasmid pSERB1. This plasmid in the Bhutanese S. sonnei incorporated the type IV pilus biosynthesis cluster pilL–V (Dudley et al., 2006). For the S. sonnei containing the IncK plasmid, the blaCTXM-15 gene was encoded along with the fluoroquinolone resistance enhancement gene qnrS1 on an IS26 transposon. The S. sonnei IncI1 plasmid (denoted pHUSEC2011-1-like) was almost identical ( ≥ 99 % DNA sequence identity) to the replication, conjugation and type IV pilus biosynthesis cluster pilL–V in pHUSEC2011-1. The pHUSEC2011-1-like plasmid was also independently acquired by six additional strains, although in these cases the blaCTXM-15 gene was not co-transferred (Fig. 2).