Fitness Advantage of -Bearing IncI2 and IncX4 Plasmids

The objective of this study was to assess the impact of diverse plasmids bearing colistin resistance gene on host fitness. Forty-seven commensal isolates recovered from the pig farm where was first identified were screened for -bearing plasmids were characterized by sequencing. The fitness impact of -bearing plasmids was evaluated by competition assays. Twenty-seven (57.5%) isolates were positive for . The genes were mainly located on plasmids belonging to IncI2 ( = 5), IncX4 ( = 11), IncHI2/ST3 ( = 8), IncFII ( = 2), and IncY ( = 2). InHI2 plasmids also carried other resistance genes (, and ) and were only detected in isolates from nursery pigs. Sequences of the representative -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 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 in this farm. Increased fitness or co-selection by other antimicrobials might contribute to the further dissemination of the three epidemic -positive plasmids (IncI2, IncX4, and IncHI2) in this farm and worldwide.