Characterisation of a cointegrate plasmid harbouring blaNDM-1 in a clinical Salmonella Lomita strain.

This study aimed to characterise the molecular events underlying formation and evolution of a cointegrate plasmid harbouring blaNDM-1 and blaCMY-2 in a clinical Salmonella Lomita (S. Lomita) strain. The Salmonella strain SL131 was found to harbour two multidrug resistant (MDR) plasmids. One plasmid, pSL131_IncHI2, is a typical IncHI2 plasmid containing blaOXA-1, catB3, arr-3, sul1, qnrB4 and blaDHA-1 in a complex class 1 integron. The other plasmid, pSL131_IncA/C-IncX3, is a blaNDM-1-bearing cointegrate plasmid consisting of IncX3 and IncA/C backbones, the formation of which is mediated by IS26. Stability assay showed that the cointegrate plasmid was highly stable in its natural host - S. Lomita - but would readily resolve into single plasmids upon conjugation, during which the IncX3 blaNDM-1-bearing plasmid could be transferred to Escherichia coli strain EC600. Plasmid evolution through integration of two or more MDR plasmids would not only expand the resistance profile of the resultant plasmid, but also broaden the host spectrum of such resistance-encoding mobile elements. Better understanding of the underlying and triggering mechanisms of cointegration may facilitate development of intervention measures to curb formation and dissemination of such elements.