Department of Medical Biochemistry and Microbiology, Uppsala University, Biomedical Center Box 582, Husargatan 3, S-75123 Uppsala, Sweden.
J Antimicrob Chemother. 2017 Nov 1;72(11):3016-3024. doi: 10.1093/jac/dkx270.
To determine whether the spectrum of mutations in marR in ciprofloxacin-resistant clinical isolates of Escherichia coli shows evidence of selection bias, either to reduce fitness costs, or to increase drug resistance. MarR is a repressor protein that regulates, via MarA, expression of the Mar regulon, including the multidrug efflux pump AcrAB-TolC.
Isogenic strains carrying 36 different marR alleles identified in resistant clinical isolates, or selected for resistance in vitro, were constructed. Drug susceptibility and relative fitness in growth competition assays were measured for all strains. The expression level of marA, and of various efflux pump components, as a function of specific mutations in marR, was measured by qPCR.
The spectrum of genetic alterations in marR in clinical isolates is strongly biased against inactivating mutations. In general, the alleles found in clinical isolates conferred a lower level of resistance and imposed a lower growth fitness cost than mutations selected in vitro. The level of expression of MarA correlated well with the MIC of ciprofloxacin. This supports the functional connection between mutations in marR and reduced susceptibility to ciprofloxacin.
Mutations in marR selected in ciprofloxacin-resistant clinical isolates are strongly biased against inactivating mutations. Selection favours mutant alleles that have the lowest fitness costs, even though these cause only modest reductions in drug susceptibility. This suggests that selection for high relative fitness is more important than selection for increased resistance in determining which alleles of marR will be selected in resistant clinical isolates.
确定在对环丙沙星耐药的大肠杆菌临床分离株中 marR 突变的频谱是否存在选择偏向，无论是为了降低适应性成本，还是为了提高耐药性。MarR 是一种阻遏蛋白，通过 MarA 调节 Mar 调控子的表达，包括多药外排泵 AcrAB-TolC。
构建了携带在耐药临床分离株中发现的 36 种不同 marR 等位基因或体外选择耐药的同基因菌株。所有菌株的药物敏感性和生长竞争试验中的相对适应性均进行了测量。通过 qPCR 测量 marA 的表达水平以及各种外排泵成分作为 marR 特定突变的函数。
临床分离株中 marR 遗传改变的频谱强烈偏向于非失活突变。一般来说，在临床分离株中发现的等位基因比体外选择的突变赋予了更低的耐药性和更低的生长适应性成本。MarA 的表达水平与环丙沙星的 MIC 密切相关。这支持了 marR 突变与对环丙沙星敏感性降低之间的功能联系。
在环丙沙星耐药的临床分离株中选择的 marR 突变强烈偏向于非失活突变。选择有利于具有最低适应性成本的突变等位基因，尽管这些突变仅导致药物敏感性的适度降低。这表明，在确定哪些 marR 等位基因将在耐药的临床分离株中被选择时，相对适应性的选择比增加耐药性的选择更为重要。
