Characterization of Highly Prevalent Plasmids Coharboring mcr-1, oqxAB, and blaCTX-M and Plasmids Harboring oqxAB and blaCTX-M in Escherichia coli Isolates from Food-Producing Animals in China.

The emergence and spread of multidrug resistance (MDR) plasmids carrying the colistin resistance gene mcr-1 has become a major public health concern. However, there is a paucity of data regarding the prevalence of mcr-1 plasmids concomitantly carrying blaCTX-M and oqxAB, an efflux pump that confers resistance to multiple agents. In this study, we determined the prevalence and characteristics of plasmids coharboring mcr-1, oqxAB, and blaCTX-M as well as those harboring oqxAB and blaCTX-M in Escherichia coli from food-producing animals. We isolated 493 E. coli strains, and mcr-1, blaNDM, and blaCTX-M were present in 140 (28.4%), 51 (10.3%), and 195 (39.6%) of the isolates, respectively. The two most prevalent plasmid-mediated quinolone resistance genes were oqxAB (34.5%) and qnrS (29.4%). Nine IncHI2/ST3 plasmids co-carrying mcr-1, oqxAB, and blaCTX-M were found, and similar IncHI2/ST3 plasmids mediated dissemination of these resistance genes. Two sequenced MDR IncHI2/ST3 plasmids coharboring mcr-1, oqxAB, and blaCTX-M showed high similarity to reference plasmid pHNSHP45-2, although they were from different regions in China. Colocalization of oqxAB and blaCTX-M on the same plasmid was found in 28 isolates, including the nine plasmids harboring mcr-1. The co-dissemination of oqxAB and blaCTX-M was mediated by diverse F33:A-:B- plasmids and similar IncHI2/ST3 plasmids. Pulsed-field gel electrophoresis and multilocus sequence typing analysis of donor isolates revealed heterogeneous patterns indicating that clonal dissemination was unlikely. The high incidence of similar IncHI2/ST3 plasmids simultaneously possessing mcr-1, oqxAB, and blaCTX-M poses a great threat to public health.