Fitness Advantage of mcr-1–Bearing IncI2 and IncX4 Plasmids in Vitro

Abstract

The objective of this study was to assess the impact of diverse plasmids bearing colistin resistance gene <i>mcr-1</i> on host fitness. Forty-seven commensal <i>E. coli</i> isolates recovered from the pig farm where <i>mcr-1</i> was first identified were screened for <i>mcr-1. mcr-1</i>-bearing plasmids were characterized by sequencing. The fitness impact of <i>mcr-1</i>-bearing plasmids was evaluated by <i>in vitro</i> competition assays. Twenty-seven (57.5%) <i>E. coli</i> isolates were positive for <i>mcr-1</i>. The <i>mcr-1</i> genes were mainly located on plasmids belonging to IncI2 (<i>n</i> = 5), IncX4 (<i>n</i> = 11), IncHI2/ST3 (<i>n</i> = 8), IncFII (<i>n</i> = 2), and IncY (<i>n</i> = 2). InHI2 plasmids also carried other resistance genes (<i>floR, bla</i>_CTX-M, and <i>fosA3</i>) and were only detected in isolates from nursery pigs. Sequences of the representative <i>mcr-1</i>-bearing plasmids were almost identical to those of the corresponding plasmid types reported previously. An increase in the fitness of IncI2- and IncX4-carrying strains was observed, while the presence of IncHI2, IncFII and IncY plasmids showed a fitness cost although an insignificant fitness increase was initially observed in IncFII or IncY plasmids-containing strains. Acquisition of IncI2-type plasmid was more beneficial for host <i>E. coli</i> DH5α than either IncHI2 or IncX4 plasmid, while transformants with IncHI2-type plasmid presented a competitive disadvantage against IncI2 or IncX4 plasmid containing strains. In conclusion, IncI2, IncX4, and IncHI2 were the major plasmid types driving the dissemination of <i>mcr-1</i> in this farm. Increased fitness or co-selection by other antimicrobials might contribute to the further dissemination of the three epidemic <i>mcr-1</i>-positive plasmids (IncI2, IncX4, and IncHI2) in this farm and worldwide.