Tn6026 and Tn6029 are found in complex resistance regions mobilised by diverse plasmids and chromosomal islands in multiple antibiotic resistant Enterobacteriaceae

•Tn6026 and Tn6029 are widely disseminated on plasmids in Enterobacteriaceae sp.•Tn21 mercury resistance transposons are associated with mobility of Tn6026/6029.•IS26 plays a significant role in the evolution of these transposons.•These transposons provide a scaffold for the acquisition of further resistance genes.

Transposons flanked by direct copies of IS26 are important contributors to the evolution of multiple antibiotic resistance. Tn6029 and Tn6026 are examples of composite transposons that have become widely disseminated on small and large plasmids with different incompatibility markers in pathogenic and commensal Escherichia coli and various serovars of Salmonella enterica. Some of the plasmids that harbour these transposons also carry combinations of virulence genes. Recently, Tn6029 and Tn6026 and derivatives thereof have been found on chromosomal islands in both established and recently emerged pathogens. While Tn6029 and Tn6026 carry genes encoding resistance to older generation antibiotics, they also provide a scaffold for the introduction of genes encoding resistance to a wide variety of clinically relevant antibiotics that are mobilised by IS26. As a consequence, Tn6029 and Tn6026 or variants are likely to increasingly feature in complex resistance regions in multiple antibiotic resistant Enterobacteriaceae that threaten the health of humans and food production animals.