Impact of plasmid-borne oqxAB on the development of fluoroquinolone resistance and bacterial fitness in Escherichia coli.
To investigate the impact of plasmid-borne oqxAB genes on the development of fluoroquinolone resistance, mutations and bacterial fitness in Escherichia coli .



MICs and mutation prevention concentrations were compared among E. coli strain TOP10 and two corresponding transformants harbouring the OqxAB-encoding plasmids. Mutants were selected by serial passages with the 0.5-fold MIC of ciprofloxacin, and were randomly selected to determine mutations. Bacterial fitness was evaluated by competition assays in vitro and in vivo .



The oqxAB -carrying plasmids contributed to a 4-8-fold increase in the ciprofloxacin MIC and increased the ciprofloxacin mutation prevention concentration by 8-16-fold. The MIC of ciprofloxacin for the two transformants increased faster than that of E. coli TOP10 by serial passaging. Novel mutations in gyrB (A468P or F458V) were first observed. Mutations in gyrA were distributed at codons 87 and 83 in the two transformants, whereas mutation A119E in gyrA dominated in the TOP10 mutants. Although the two oqxAB -bearing plasmids caused a decrease in fitness in vitro , their fitness increased when combined with more than one chromosomal mutation, and clear biological benefits were observed in vivo . The mutations in gyrB were associated with a fitness cost, which could be compensated for by additional mutations. The novel mutation gyrA ΔS83 significantly reduced biological fitness both in vitro and in vivo , and was thus quickly replaced by more beneficial mutations in the population.



The possession of plasmid-borne oqxAB may facilitate the evolution of fluoroquinolone resistance, and the fitness cost of OqxAB-encoding plasmids could be compensated by additional chromosomal mutations.
Jing
J
Wang
Ze-Wen
ZW
Guo
Chan-Ping
CP
Zhi
Tong
T
Yang
Jing-Jing
JJ
Zhao
Xiao-Jie
XJ
Chen
Li
L
Zeng
Lu-Chao
LC
Lv
Zhen-Ling
ZL
Zeng
Jian-Hua
JH
Liu
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003062:Codon; D027081:DNA Gyrase / Q000235:genetics; D024881:Drug Resistance, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000187:drug effects* / Q000235:genetics*; D024841:Fluoroquinolones / Q000494:pharmacology*; D005798:Genes, Bacterial; D019450:Genes, MDR; D056084:Genetic Fitness; D006801:Humans; D008826:Microbial Sensitivity Tests; D009154:Mutation; D010957:Plasmids
D000900:Anti-Bacterial Agents; D003062:Codon; D024841:Fluoroquinolones; D027081:DNA Gyrase