The evolution and transmission of multi-drug resistant Escherichia coli and Klebsiella pneumoniae: the complexity of clones and plasmids.

The vast majority of Escherichia coli and Klebsiella pneumoniae isolated from human clinical extra-intestinal infections are now multi-drug resistant (MDR). Extended Spectrum Beta Lactamase (ESBL) carriage in clinical isolates of these bacteria is now commonplace, and carriage of carbapenemases is continuing to increase. MDR is primarily concentrated in a small number of globally disseminated clones, which generally differ between ESBL and carbapenemase carrying-clones in E. coli, but seem to converge in K. pneumoniae. In both species MDR is mediated by acquisition and maintenance of MDR plasmids. The plasmids associated with ESBL and carbapenemases also differ, and when both resistances are present in the same strain they are generally on distinct plasmids. Recent research is attempting to provide clues as to why some lineages appear better suited to acquisition and maintenance of these plasmids without a fitness cost. Central to this is the appearance of adaptive mutations in intergenic regions, and selection on genes involved in anaerobic metabolism, hinting at a process whereby these clones can outcompete commensal strains of the same species to initiate long-term intestinal colonization.