AUTHOR=Wu Renjie , Yi Ling-xian , Yu Lin-feng , Wang Jing , Liu Yiyun , Chen Xiaojie , Lv Luchao , Yang Jun , Liu Jian-Hua 

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

JOURNAL=Frontiers in Microbiology

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2018.00331

DOI=10.3389/fmicb.2018.00331

ISSN=1664-302X

ABSTRACT=<p>The objective of this study was to assess the impact of diverse plasmids bearing colistin resistance gene <italic>mcr-1</italic> on host fitness. Forty-seven commensal <italic>E. coli</italic> isolates recovered from the pig farm where <italic>mcr-1</italic> was first identified were screened for <italic>mcr-1. mcr-1</italic>-bearing plasmids were characterized by sequencing. The fitness impact of <italic>mcr-1</italic>-bearing plasmids was evaluated by <italic>in vitro</italic> competition assays. Twenty-seven (57.5%) <italic>E. coli</italic> isolates were positive for <italic>mcr-1</italic>. The <italic>mcr-1</italic> genes were mainly located on plasmids belonging to IncI2 (<italic>n</italic> = 5), IncX4 (<italic>n</italic> = 11), IncHI2/ST3 (<italic>n</italic> = 8), IncFII (<italic>n</italic> = 2), and IncY (<italic>n</italic> = 2). InHI2 plasmids also carried other resistance genes (<italic>floR, bla</italic><sub>CTX−M</sub>, and <italic>fosA3</italic>) and were only detected in isolates from nursery pigs. Sequences of the representative <italic>mcr-1</italic>–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 <italic>E. coli</italic> 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 <italic>mcr-1</italic> in this farm. Increased fitness or co-selection by other antimicrobials might contribute to the further dissemination of the three epidemic <italic>mcr-1</italic>–positive plasmids (IncI2, IncX4, and IncHI2) in this farm and worldwide.</p>